:j  THE 

PHYSICAL  CHEMISTRY 
OF  THE  METALS 


BY 

RUDOLPH    SCHENCK 

•  J 

Professor  of  Physical  Chemistry  in  the  Technischen  Hochschule 

in  Aachen 


TRANSLATED  AND  ANNOTATED  BY 

REGINALD    SCOTT    DEAN 

Research  Metallurgist,  American  Zinc,  Lead  and  Smelling  Co., 
St.  Louis,  Mo. 


FIRST    EDITION 


NEW  YORK 

JOHN   WILEY   &   SONS,    INC. 

LONDON:  CHAPMAN  &  HALL,  LIMITED 

1919 


COPYRIGHT,  1919,  BY 
REGINALD  S.  DEAN 


PRESS  OF 

BRAUNWORTH   &  CO. 

BOOK  MANUFACTURERS 

BROOKLYN,   N.   Y, 


PREFACE 


THERE  is  perhaps  no  field  where  physical  chemistry  is  of  more 
value  than  in  the  field  of  metallurgy  and  metallography.  In 
the  latter,  field  its  usefulness  has  been  recognized  almost  from 
the  first  but  in  the  field  of  metallurgy  its  application  has  not 
been  so  general.  It  is  hoped  that  this  translation  may  aid  in 
making  the  value  of  chemical  dynamics  and  equilibrium  clear 
to  the  metallurgist  and  metallurgical  student. 

Such  additions  as  have  seemed  necessary  have  been  incor- 
porated in  the  text  and  the  numerical  data  have  been  revised  to 
agree  with  the  accepted  values.  I  have  deemed  it  advisable  not 
to  go  into  the  recent  investigations  concerning  the  electron 
theory  since  the  scope  of  the  work  did  not  seem  to  warrant  an 
extended  treatise  on  this  subject. 

The  book  has  been  changed  from  lecture  to  text-book  form 
and  the  references  shifted  from  the  appendix  to  the  body  of  the 
book. 

My  thanks  are  due  to  Mr.  A.  T.  McPherson  of  the  U.  S. 
Bureau  of  Standards  for  reading  the  manuscript  as  well  as  for 
many  valuable  suggestions.  I  wish  also  to  thank  Dr.  Edward 
Schramm,  director  of  this  laboratory,  for  his  .encouragement  and 
cooperation  in  the  work  of  translation. 

REGINALD  S,  DEAN. 

ST.  Louis,  Mo. 
March,  1919. 


111 


PREFACE  TO  THE  GERMAN  EDITION 


THIS  little  book  is  the  outcome  of  a  series  of  lectures  which  I 
delivered  in  1907,  in  the  "  Technischen  Hochschule,"  at  Aachen. 
Their  purpose  was  to  show  the  engineers  of  the  Rhenish  indus- 
trial district,  before  whom  they  were  delivered,  the  use  of  chem- 
ical statics  and  to  deepen  their  understanding  of  smelting  opera- 
tions and  metallurgical  processes. 

I  have  endeavored,  especially,  to  develop  the  principles  of 
equilibrium  clearly  and  so  far  as  possible  by  the  use  of  per- 
tinent examples. 

In  the  systematic  survey  of  such  a  field  new  problems  nat- 
urally arise  and  there  is  found  in  these  lectures  some  heretofore 
unpublished  data  bearing  on  these  problems,  among  which  may 
be  mentioned:  the  equilibrium  between  the  various  components 
of  steel,  the  quantitative  determination  of  amorphous  carbon 
and  graphite,  and  the  investigation  of  the  sulfatizing  roast. 

It  has  not  been  possible  to  consider  here,  all  of  the  experi- 
mental matter  relating  to  the  physical  chemistry  of  metals,  but 
all  fundamental  questions  have  been  treated  rather  thoroughly. 

I  wish  to  thank  Dr.  P.  Goerens,  for  the  preparation  of  the 
metallographs  and  photographs;  Dr.  Hemplemann,  for  his 
assistance  in  the  preparation  of  the  index,  and  Dr.  Ratzbach, 
for  the  preparation  of  the  diagrams. 

THE  AUTHOR. 

AACHEN. 
July,  1908. 


CONTENTS 


PREFACE iii 

PREFACE  TO  GERMAN  EDITION v 

CHAPTER  I 
INTRODUCTION,  PROPERTIES  OF  METALS       1-38 

Introduction,  i.  Form  Changes,  3.  Vapor  Pressure  and  Volatility,  3. 
Monatomic  State  of  Metal  Vapors,  5.  Color  of  Metal  Vapors,  6.  Vapor 
Pressure  and  the  Definition  of  Melting  Point,  7.  Melting  Points  of 
Metals,  8.  Density  Change  on  Melting,  9.  Heat  of  Fusion,  9.  Cooling 
Curves,  10.  Polymorphy,  n.  Analogy  of  Polymorphic  Transition 
to  Melting,  14.  Determination  of  the  Transition  Point,  14.  Explosive 
Antimony,  17.  Enantiotropy  and  Monotropy,  18.  Crystal  Growth, 
20.  Electrical  and  Optical  Properties  of  Metals,  Conductivity,  21. 
Faraday's  Law,  22.  Metallic  and  Gaseous  Conduction,  22.  Electron 
Theory,  23.  Law  of  Wiedemann  and  Franz,  23.  Optical  Properties  of 
Metals;  Light  Reflection  and  Absorption,  27.  Metallic  Luster,  28. 
Electron  Concentration  of  Metals,  30.  Temperature  Coefficient  of 
Conductivity,  31.  Thermo-Electric  Force,  32.  Passivity  in  Metals,  34. 

CHAPTER  II 
METALLIC  SOLUTIONS  AND  ALLOYS 39-81 

Colloidal  Metal  Solutions,  39.  Dilute  Metallic  Solutions,  41.  Vapor 
Pressure  of  Metallic  Solutions,  42.  Freezing  Point  of  Alloys,  44.  Metals 
only  Partially  Miscible  in  the  Liquid  State,  46.  The  Parkes  .Process, 
47.  Solidification  Curve  of  Binary  Alloys,  the  System  Cadmium- 
Zinc,  49.  The  Pattison  Process,  52.  Segregation  in  Alloys,  52.  Investi- 
gation of  Alloy  Structure,  53.  Solid  Solutions,  55.  The  Heraeus  Process 
of  Plating  Platinum  with  Gold,  57.  Crystallization  Diagrams  for 
Metals  Forming  Solid  Solutions,  57.  Inter-metallic  Compounds,  60. 
Ternary  and  Quaternary  Alloys,  64.  Relations  of  Mechanical  and 
Physical  Properties  to  Alloy  Structure,  68.  Bearing  Metals,  69.  Den- 
sity of  Alloys,  70.  Potential  of  Alloys,  72.  Electrical  Resistance  of 
Alloys,  73.  Thermo-electric  Force  of  Alloys,  80. 

vii 


viii  CONTENTS 


CHAPTER  III 

PAGE 

ALLOYS  OF  METALS  WITH  CARBIDES,  OXIDES  AND  SULFIDES,  IRON  AND 

STEEL,  MATTES,  PHASE  RULE ' 82-138 

Compounds  with  Metallic  Properties,  82.  The  Iron — Carbon  Alloys, 
83.  The  Crystallization  of  the  System  Iron — Cementite,  85.  Perlite 
and  its  Transition  Products,  93.  The  Precipitation  of  Carbon  in 
Iron — Carbon  Alloys,  98.  The  Uses  of  Additions  to  Iron  and  Steel, 
104.  Alloys  of  Metals  and  Oxides;  Copper — Copper  Oxide,  no. 
Silver — Silver  Oxide,  113.  Alloys  of  Metals  and  Sulfides,  114. 
Lead — Lead  Sulfide,  115.  Antimony — Antimony  Sulfide,  115.  — 
Copper  Copper  Sulfide,  116.  Silver — Silver  Sulfide,  118.  Iron — 
Iron  Sulfide,  120.  Nickel — Nickel  Sulfide,  122.  Alloys  between 
Sulfides,  123.  Phosphorus  and  Arsenic  Containing  Alloys,  125. 
Silicides  of  Metals,  131.  The  Phase  Rule,  133. 


CHAPTER  IV 
THE  METALLURGICAL  REACTIONS,  OXIDATION  AND  REDUCTION 139-169 

Equilibrium  between  Metal,  Oxide  and  Oxygen,  140.  The  Applica- 
tion of  the  Phase  Rule,  140.  Oxygen  Tension  of  Oxides,  141. 
Oxygen  Tension  in  Atmospheres,  143.  The  Equilibrium  between  two 
Oxides  and  Oxygen,  145.  Direct  Decomposition  of  Oxides  by 
Heat,  148.  Reduction  by  Metals,  149.  Reduction  by  Gaseous 
Reducing  Agents,  149.  Reduction  by  Hydrogen,  150.  The  Phase 
Rule,  150.  LeChatelier's  Principle,  151.  The  Mass  Law,  152. 
Van't  Hoff's  Equation,  156.  Reduction  by  Carbon  Monoxide,  159. 

CHAPTER  V 
DECOMPOSITION  OF  CARBON  MONOXIDE,  BLAST  FURNACE  PROCESS 170-204 

Catalytic  Decomposition  of  Carbon  Monoxide,  1 70.  Equilibrium 
between  Carbon  Monoxide  and  Iron,  173.  The  Analysis  of 
Mixtures  of  Graphite  and  Amorphous  Carbon,  178.  The  Blast 
Furnace  Process,  180.  The  Mond  Nickel  Process,  202. 

CHAPTER  VI 
THE  REACTIONS  OF  SULFIDES      205-227 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


CHAPTER   I 
INTRODUCTION,  PROPERTIES  OF  METALS 

IT  is  a  well-known  fact  that  the  metal  industry  is  based 
largely  on  chemical  knowledge.  The  metals  were  the  first 
chemically  defined  substances  to  be  recognized,  and  the  first  to 
be  obtained  by  chemical  means;  it  was  thus  that  man  acquired 
the  beginning  of  his  hard-earned  chemical  knowledge. 

What  a  heritage  of  facts  has  come  to  the  present  day  chemist 
simply  through  the  efforts  of  the  alchemist  to  change  base  metals 
into  noble.  Interest  in  the  chemistry  of  metals  has  never 
lagged;  the  mines  and  the  pits  of  the  furnaces  were  to  Berzelius 
and  many  other  chemists  the  inspiration  to  notable  new  investi- 
gations; the  application  of  new  agents,  particularly  the  voltaic 
battery,  in  the  electric  decomposition  of  salts,  have  brought  to 
light  new  metals  with  remarkable  properties,  for  example, 
sodium,  potassium,  aluminium,  magnesium  and  many  more. 
The  names  of  Davy,  Wohler,  and  Winkler  recall  to  our  minds 
what  great  advances  the  chemistry  of  metals  has  made  in  the 
past  century.  These  include  the  discovery  not  only  of  new 
metals  but  also  of  the  methods  by  which  mineral  substances 
are  recognized  and  their  amounts  determined  with  certainty. 

Technical  ch.emists,  especially  metallurgists,  are  already 
convinced  of  the  value  of  chemical  analysis,  not  only  in  the  in- 
vestigation of  ores,  but  in  the  control  of  processes.  Experience 
has  shown  that  the  success  or  failure  of  a  smelter,  may,  under 


2  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

certain  conditions,  depend  on  the  reliability  of  its  analyst.  The 
introduction  of  chemical  analysis  into  smelters  was  the  first 
great  advance  toward  rational  operation. 

In  the  course  of  the  last  century  the  machine  industry  and  the 
electrical  industry  have  made  unheard-of  progress;  the  metal- 
lurgical industries,  especially  that  of  iron  and  steel,  are  now 
growing  into  a  sphere  of  new  problems.  The  purchaser  demands 
not  only  large  amounts  of  material  at  the  lowest  possible  price, 
but  he  also  places  entirely  new  and  definite  demands  on  the 
quality  of  the  same.  Bridge  and  machine  builders  need  steel 
of  definite  mechanical  properties,  the  electrical  industry  needs 
steel  of  definite  magnetic  properties,  the  tool  manufacturer,  a 
steel  that  retains  its  hardness  when  hot,  ships  and  machines, 
needles  and  pins,  wire  and  sheet,  cook  stoves  and  furnaces  all 
are  made  of  iron  and  each  one  requires  an  entirely  different  mate- 
rial. Iron  must  play  the  part  of  a  veritable  Proteus  to  possess 
all  these  properties  which  are  often  diametrically  opposite.  It 
is  astounding  that  one  metal  is  endowed  with  so  great  a  varia- 
tion in  its  properties  and  it  is  remarkable  that  the  industries 
have  found  a  use  for  this  material  in  each  of  its  notable  properties. 

The  growth  of  our  metal  industries  is  largely  due  to  machine 
and  bridge  building,  since  the  carrying  out  of  a  large  number 
of  these  projects  required  appropriate  material  and  the  smelter- 
man  was  thereby  directed  to  experiments  and  tests  for  their 
production. 

Many  problems  in  metallurgy  cannot  be  solved  by  means  of 
analytical  chemistry,  since  it  is  often  found  that  material  of 
basically  different  properties  gives  identically  the  same  analysis 
and  that  extraordinary  relations  in  physical  properties  are 
afforded  no  explanation  by  the  analysis. 

Structural  chemistry,  as  developed  for  organic  compounds, 
also  fails  to  be  of  value  here,  and  accordingly  the  only  resort  was 
the  purely  empirical.  Every  empirical  process  that  we  know 
works  uneconomically;  a  success  follows  ten  failures;  every  suc- 
cessful advance  is  costly  in  experience  and  money. 

All  the  phases  of  metallurgy  have  not  yet  been  placed  on  a 
scientific  basis,  but  some  of  the  obscurities  have  been  disposed 


INTRODUCTION— PROPERTIES  OF  METALS  3 

of  and  progress  made  in  the  solution  of  some  of  the  problems 
presented  by  the  smelting  art.  This  has  been  accomplished 
by  means  of  physical  chemistry;  especially  through  chemical 
mechanics  which  for  metallurgical  engineers  and  technical  chem- 
ists possesses  the  same  importance  that  physical  mechanics  does 
for  bridge  and  machine  builders. 

We  will  now  attempt  with  the  examples  at  hand  to  present  a 
picture  of  the  manner  in  which  the  problems  of  metallurgy  are 
solved  with  the  help  of  physical  chemistry. 

We  will  not  consider  the  electrochemical  relations  of  the 
metals  as  electro-metallurgy  has  long  since  left  the  scientific 
nursery,  gone  out  into  life  and  proved  its  worth. 

We  may  not,  however,  overlook  the  youngest  branch  of 
electrical  science,  the  electron  theory  of  metals,  since  it  explains 
many  of  the  remarkable  physical  peculiarities  of  the  metallic 
state. 

Form  Changes. 

We  must  first  consider  certain  physical  phenomena  which  the 
metals  show  in  common  with  non-metallic  substances  but  which 
play  an  especially  important  role  in  the  working  of  the  metals. 
They  are  the  form  changes  which  the  metals  undergo  at  high 
temperatures,  the  phenomena  of  vaporization,  of  melting,  and 
the  polymorphic  transitions  in  the  solid  state.  For  the  present 
we  shall  limit  ourselves  to  the  pure  metals,  later  taking  up  solu- 
tions and  mixtures. 

Vapor  Pressure  and  Volatility. 

A  number  of  metals  go  over  easily  to  the  gaseous  state.  The 
volatility  of  mercury  is  detectable  at  atmospheric  temperatures 
and  cadmium  and  zinc  are  obtained  by  the  use  of  a  distillation 
process.  The  boiling  points  of  these  metals  are  relatively  low. 
At  high  temperatures  such  as  can  be  obtained  in  the  oxy-hydrogen 
flame,  the  less  volatile  metals  are  vaporized.  It  is  known  that 
silver  distils  at  such  temperatures  since  Stas  in  his  classical 
investigations  on  atomic  weights  purified  silver  in  this  way. 
At  the  highest  temperature  of  the  electric  arc  there  is  no  metal 
that  does  not  vaporize. 


4 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


Moissan  *  succeeded  with  the  help  of  a  strong  current  in 
boiling  and  distilling  most  metals;  copper  and  gold  as  well  as 
platinum,  palladium,  iridium,  rhodium,  ruthenium,  and  osmium 
can  be  vaporized  and  precipitated  again  on  a  cooled  metal  sur- 
face. Of  the  iron  group,  manganese  is  known  to  be  the  most 
volatile,  then  nickel  and  then  chromium.  Iron  itself  boils  with 
relative  difficulty  and  the  boiling  points  of  uranium,  molybdenum 
and  tungsten  are  still  higher  but  they  are  undoubtedly  volatile 
in  an  electric  arc. 

Many  metals  that  are  difficultly  vaporized  at  ordinary  pres- 
sures are  easily  brought  to  vaporization  and  distillation  under 
strongly  reduced  pressure.  Krafft  and  Kahlbaum  f  and  Roth 
and  Siedler  J  have  made  investigations  of  this  kind.  The  latter 
have  purified  gold,  silver,  copper,  lead,  bismuth  and  antimony 
by  distillation  in  the  highest  obtainable  vacuum  and  investi- 
gated a  series  of  properties  of  the  metals  so  purified. 

The  following  table  gives  the  approximate  values  for  the  boil- 
ing  points  of  metals  so  far  as  they  have  been  determined.  The 
values  are  those  given  by  J.  Johnston,  J.  Ind.  Eng.  Chem.  9, 
873  (1917),  unless  otherwise  noted. 


Metal. 

Boiling  Point.* 
Degrees  C 

Metal. 

Boiling  Point.* 
Degrees  C 

Cadmium 

?8o 

Chromium 

22OO 

Zinc  

Q2O 

Tin  

226o 

Magnesium  

1  1  2O 

Copper  . 

2?  CO 

Thallium  . 

1  3OO 

Nickel  

2J.OO 

Bismuth 

I4.4.O 

Iron 

24.C.O 

Antimony  

I44O 

Arsenic  

610 

Lead 

1640 

Platinum  f  .  .  . 

3QO7 

Aluminum  
Manganese 

I800 
IOOO 

Molybdenum  f  

Tungsten  t 

3617 

4827 

Silver  

2OQO 

Sodium  J  

742 

Mercurv  1.  . 

3C7.  2C 

*  i.e.,  the  temperature  at  which  the  vapor  pressure  is  760  mm. 
t  Langmuir,  Phys.  Rev.  2,  329  (1913). 
j  From  Schenck's  table. 


*  Compt.  rend.,  141,  853:  977  (1905). 

t  Ber.  36,  1690  (1903). 

JZ.  anorg.  Chem.,  29,  177  (1902). 


142,  189;  425;  673  (1906). 


INTRODUCTION— PROPERTIES  OF  METALS 


For  volatile  substances  every  temperature  corresponds  to  a 
definite  vapor  pressure  which  increases  with  rising  temperature. 
The  relation  between  the  two  has  been  investigated  by  Barus  and 
others.  The  approximate  boiling  temperatures  of  metals  at 
various  pressures,  i.e.,  the  temperature  at  which  their  vapor 
pressure  is  io~3  mm.,  etc.,  is  given  in  the  following  table  taken 
from  Johnston. 


Metal. 

p  IN  MM.  MERCURY. 

io-3 

io-2 

10   ' 

I 

10 

So 

100 

760 

Cadmium  .  . 

°C 
220 
2QO 
380 
500 
540 
540 
62O 
730 
7QO 
Q20 
980 
IOIO 

1080 

IIOO 

1130 

220 

°C 
270 

35° 
440 

570 
620 
620 
710 
830 
890 
1030 
1090 
1130 

1200 
I22O 
1250 
260 

°C 
330 
420 
520 
660 
720 
720 
820 
950 

IO2O 

1160 

1230 

1270 

1340 
1370 

1400 

310 

°c 
410 

500 

620 

770 
840 
840 

960 
1090 
1170 
1320 
1400 
1440 
1520 
1550 
1590 
360 

°C 
500 
610 

750 
910 
990 
990 
1130 
1280 
1360 
3520 
1610 
1660 
1740 
1780 
1820 
430 

°C 
590 
700 
860 
1030 
1130 
1130 
1290 
1440 

1530 
1700 
1800 
1850 
1930 
1970 
2OIO 
490 

°C 
630 

750 
920 
1090 
1  200 

1200 
1360 
1520 

1610 
1780 
1890 
1940 
2030 
2070 

2IIO 
5!0 

°C 
780 
920 
1120 
1300 
1440 
1440 
1640 
I800 
1900 
2O9O 
2200 
2260 

2350 
2400 

2450 

610 

Zinc  

Magnesium  .  .  . 

Thallium 

Bismuth  

Antimony  . 

Lead  

Aluminum 

Manganese  

Silver 

Chromium   

Tin  

CoDoer 

Nickel  

Iron       

Arsenic 

If  the  relation  of  the  vapor  pressure  to  the  temperature  for 
any  metal  be  represented  graphically,  a  curve  is  obtained  which 
corresponds  to  the  vapor  pressure  curve  of  non-metallic  sub- 
stances, for  example,  water.  (Fig.  i.) 

Monatomic  State  of  Metal  Vapors. 

In  calculating  the  molecular  weight  from  the  vapor  density 
of  metals  it  is  seen  that  the  molecules  consist  of  only  one  atom, 
differing  in  this  respect  from  most  other  substances.  The 
monatomic  state  of  the  vapor  has  been  established  not  only 
for  the  easily  vaporized  metals,  mercury,  cadmium  and  zinc 
(Victor  Meyer)  *  but  also  for  the  difficultly  vaporizable  metals 

*  Ber.  12,  1426  (1879). 


6 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


bismuth,  antimony,  lead,  and  thallium  (von  Wartenberg.*) 
The  monatomic  state  is  also  revealed  by  another  circumstance. 
Gases  have  recognizedly  two  different  specific  heats.  The 
amount  of  heat  which  we  must  bring  into  a  gas  in  a  closed 
constant  volume  to  raise  it  to  a  higher  temperature  is  smaller 
than  that  required  to  accomplish  the  heating  at  constant  pres- 
sure. In  the  latter  case  the  gas  increases  in  volume  and  thereby 
performs  work.  It  can  be  deduced  from  the  kinetic  theory 


800m  m. 


700 


700 


600 


600' 


500 


500 


400 


400 


300' 


300 


200 


200 


100 


100 


200°    300°  500°  700°  900  1100°  1300C 

Vapor  Pressure  of  the  Metals  Hg ,  Cd  >  Zn  and  Bi 
FIG.  i. 


1500 c 


that  for  a  monatomic  gas  the  ratio  of  the  specific  heat  at  con- 
stant pressure  to  that  at  constant  volume  must  have  a  definite 
value,  namely  1.667.  Kundt  and  Warburg  f  found  the  value 
1.666  fpr  mercury  vapor.  Besides  the  metals  only  the  rare 
gases  of  the  atmosphere,  argon,  xenon,  helium,  krypton,  and 
neon  are  monatomic.  The  molecules  of  all  other  substances 
are  polyatomic. 

Color  of  Metal  Vapors. 

The  vapors  of  the  metals  often  show  an  intense  coloring 
which  is  especially  easy  to  observe  with  potassium  vaporized 

*  Z.  anorg.  Chem.,  56,  320  (1907).  f  Pogg.  Ann.,  157,  353  (1878). 


INTRODUCTION— PROPERTIES  OF  METALS 


in  an  atmosphere  of  hydrogen.  The  color  of  the  vapor  is  in- 
tensely green.  Sodium  vapor  is  blue.  It  is  the  powerful 
absorption  of  these  metal  vapors  which  gives  rise  to  the  Frauen- 
hofer  lines  in  the  solar  spectrum. 

Vapor  Pressure  and  the  Definition  of  Melting  Point. 

We  must  ascribe  to  all  substances,  the  capacity  of  sending 
off  vapor  molecules  from  their  upper  surfaces.  That  we  cannot 
observe  them  is  due  to  their  smallness  and  the  relative  insensi- 
bility of  our  methods.  Vaporization  is  not  confined  to  liquids, 
for  solids  also  vaporize  and  have  a  definite  vapor  tension.  For 
the  volatile  solid  substances 
as  iodine  and  camphor  its  mag- 
nitude can  be  readily  deter- 
mined. Substances  like  benzene 
and  naphthalene  whose  vapor 
pressures  can  be  readily  meas- 
ured in  both  the  solid  and  the 
liquid  state  have  been  investi- 
gated and  the  relation  of  vapor 
pressure  to  temperature  estab- 
lished for  both  states.  From 
this  it  has  been  found  that 
each  state  has  a  special  vapor 
pressure  curve.  It  is  seen 
from  Fig.  2  that  these  curves 
must  intersect  once  and  the 
temperature  of  this  intersec- 
tion is  no  other  than  the  melt- 
ing point.  The  melting  point 
of  a  substance  may  accordingly  be  defined  as  the  temperature 
at  which  the  vapor  pressure  of  the  solid  substance  is  equal  to 
that  of  the  liquid.  The  melting  point  is  an  equilibrium  point 
at  which  solid  and  liquid  can  exist  together.  If  heat  be  added  to 
a  substance  at  its  melting  point  the  solid  will  disappear  without 
change  of  temperature  after  which  the  temperature  will  rise. 
If  heat  be  subtracted  the  liquid  will  solidify  and  the  temperature 
will  only  fall  after  the  entire  mass  is  solid.  It  is  sometimes 


Temp. 


M.P. 
FlG.   2. 


8 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


possible,  however,  to  cool  a  liquid  below  its  melting  point  with- 
out the  separation  of  solid  taking  place.  Such  liquids  are  said 
to  be  supercooled.  The  supercooled  liquid  has  a  vapor  pressure 
curve  which  is  a  direct  continuation  of  that  part  of  the  liquid 
curve  which  lies  above  the  melting  point.  Its  vapor  pressure  is 
always  higher  than  that  of  the  solid  at  the  same  temperature, 
crystallization  is  therefore  accompanied  by  a  decrease  in  vapor 
pressure.  Above  the  melting  point  the  liquid  has  a  lower  vapor 
pressure  than  the  solid  would  possess  if  it  could  be  observed 
there.  The  transition  of  liquid  into  solid,  or  conversely  of  solid 
into  liquid  always  takes  place  in  the  direction  which  results  in  a 
decrease  of  vapor  pressure.  The  state  with  the  lower  vapor 
pressure  is  always  stable,  that  with  the  higher  always  metastable. 
At  the  point  of  equal  vapor  pressure,  that  is,  the  melting  point, 
both  states  are  equally  stable  and  there  is  no  tendency  to  change. 

Melting  Points  of  Metals. 

The  melting  points  vary  greatly  for  different  metals.     The 
most  recent  values  are  given  in  the  table  below. 


[Mercury 

'°C 

—  ^8  8? 

Barium  

°C 
850 

Caesium        

26 

Germanium  

958 

Gallium 

OQ     I 

Silver 

060  S 

Rubidium                  .  . 

38 

Gold  

1063  .  o 

Potassium 

62  3 

Copper 

1083.0 

Sodium 

07    "? 

Manganese  

1230 

Indium        

ICC 

Nickel  

1452 

1  86 

Cobalt 

14.80 

Tin 

231    0 

Chromium  

1615 

Bismuth 

271 

Iron    

IC.3O 

Thallium 

3O2 

Palladium  

1549 

•720   O 

Vanadium                .  .  . 

1720 

Lead 

•J27   4. 

Platinum  

1755 

Zinc 

4.IQ   4. 

Titanium  

1800 

Antimony 

6^O 

Uranium  

<i85o 

6<U 

Rhodium     

IQ^O 

6^8  7 

Iridium   

23C.O 

Radium  

700 

Ruthenium  

2450 

Calcium 

810 

Osmium  

2700 

Lanthanum 

870 

2550 

Strontium 

>Ca<Ba 

Tantalum  

2900 

Arsenic  

817* 

Tungsten  

354°t 

*  Goubau,  Compt.  rend.,  158,  121. 


t  Langmuir,  Phys.  Rev.,  6,  138  (1915). 


INTRODUCTION— PROPERTIES  OF  METALS 


9 


The  values  are  those  adopted  by  the  U.  S.  Bureau  of 
Standards*  except  where  recent  determinations  have  given  more 
trustworthy  figures. 

Density  Change  on  Melting. 

The  process  of  melting  is  accompanied  by  a  sudden  change  in 
physical  properties.  The  density  is  especially  to  be  considered 
in  this  connection;  its  change  on  melting  has  been  measured  for  a 
number  of  metals  as  shown  in  the  following  table  from  Landolt, 
Bornstein  and  Meyerhoffer,  Physikalische  Chemische  Tabellen.f 


Metal. 

Temp. 

DENSITY. 

Metal. 

Temp. 

DENSITY. 

Solid. 

Liquid. 

Solid. 

Liquid. 

Lead  .  . 

325 
3i8 
27 
62 

11.005 
8.366 
1.886 
0.851 

TO  045 

7.989 
1.836 
0.829 

Sodium.  .  . 
Mercury.  . 
Bismuth.  . 
Tin 

97.6 

-38.5 
271 
226 

0.9519 
14.193 
9-673 
7.183 

0.9287 
13.6902 
10.004 
6.988 

Cadmium.  . 
Caesium  .  .  . 
Potassium  . 

We  see  from  these  figures  that  the  volume  of  the  liquid  sub- 
stance is  generally  greater  than  that  of  the  solid.  There  are 
however  exceptions.  With  bismuth  the  volume  increases  on 
solidification  as  in  the  case  of  water.  These  phenomena  are 
naturally  of  great  importance  in  the  casting  of  metals  in  which 
case  it  is  necessary  to  take  into  consideration  the  density  change 
on  solidification  in  order  to  measure  rightly  the  amount  neces- 
sary to  fill  the  mold  so  that  the  cast  will  give  a  form  of  the  right 
dimensions. 

Heat  of  Fusion. 

A  definite  amount  of  heat  is  absorbed  in  the  liquefying  of  solid 
substances.  The  energy  content  of  the  liquid  is  greater  than 
that  of  the  solid  and  the  energy  difference  is  equal  to  the  added 

*Chem.  Met.  Eng.  20,  351  (1919). 

t  For  a  discussion  of  the  volume  change  of  melting  and  transition,  see  Guertler, 
J.  Inst.  Metals,  X,  193,  175.  Also  the  following  papers:  Turner,  J.  Iron  and  Steel 
Inst.,  1906, 1,  48;  Turner  and  Murray,  J.  Inst.  Metals,  No.  2,  1909,  II,  98;  Ewen 
and  Turner,  J.  Inst.  Metals,  No.  2,  1910,  IV,  128;  Houghton  and  Turner,  J.  Inst. 
Metals,  No.  2,  1911,  VI,  192;  Chamberlain,  J.  Inst.  Metals,  No.  2,  1913,  X,  193. 


10 


THE  PHYSICAL   CHEMISTRY  OF  THE  METALS 


heat  of  fusion.     These  values  are  given  in  the  following  table, 
from  Landolt,  Bornstein  and  Meyerhoffer. 


Temp. 

HEAT  OF 

FUSION. 

Temp. 

HEAT  OF 

FUSION 

°C 

i  Kg. 

ig.  At. 

°C 

i  Kg. 

ig.  At. 

Lead        .    .. 

326 

f.  If 

i  .  i 

Nickel  .  .  . 

4   6d. 

Cadmium  .... 
Iron  

320 
IOOO 

13-7 

6.0 

i-5 
o.  3 

Palladium  .  .  . 
Platinum.  .  .  . 

1500 
1770 

36.3 

27    2 

3-8 

5-2 

Gallium  

12 

19.  i 

I  .  2 

Mercury.  .  .  . 

2    82 

o  6 

Potassium  . 

58 

15.7 

0.6 

Silver  . 

000 

21    I 

2      7 

CoDDer 

43   O 

2.  7 

Bismuth 

266 

12    6 

2    6 

Sodium 

06 

7i    7 

o  7 

Zinc 

A.TL  "\ 

28  6 

i  8 

Tin  

232 

14.    2 

I    7 

Cooling  Curves. 

If  the  melt  is  allowed  to  cool  slowly  and  the  change  of  temper- 
ature with  time  observed,  it  is  seen  that  as  long  as  the  mass  is 
liquid  there  is  a  regular  fall  of  temperature;  as  soon  as  the  pre- 
cipitation of  solid  begins  the  temperature  remains  constant,  due 
to  the  liberation  of  the  latent  heat,  and  the  cooling  only  proceeds 
further  when  the  mass  has  entirely  solidified.  This  can  be  seen 
in  the  following  example  giving  the  observed  cooling  of  a  mass  of 
molten  zinc. 


Temperature 
Degrees  C 

Time 
Minutes 

Temperature 
Degrees  C 

Time 
Minutes 

480 

O 

425 

30 

470 

5 

419 

35 

461 

10 

419 

40 

452 

15 

419 

45 

443 

20 

415 

50 

434 

25 

406 

55 

397 

602 

If  these  observations  be  represented  graphically,  the  time  as 
abscissae  and  the  temperature  as  ordinates,  a  cooling  curve  is 
obtained,  from  which  the  arrest  of  cooling  during  solidification 
is  apparent.  (Fig.  3.)  In  many  cases  the  form  of  cooling  or 
solidification  curve  is  somewhat  different.  Super-cooling  phe- 


INTRODUCTION— PROPERTIES  OF  METALS 


11 


nomena  frequently  occur,  that  is,  the  solidification  does  not  begin 
when  the  melting  point  is  reached  but  the  temperature  of  the 
melt  sinks  somewhat  lower;  then  when  the  precipitation  of  the 
solid  begins,  heat  is  liberated  which  causes  a  rise  in  temperature 
till  the  melting  point  is  again  reached.  This  temperature  re- 
mains steady,  as  in  the  first  case,  until  the  crystallization  process 
is  ended.  The  form  of  the  cooling  curve  for  this  case  is  shown  in 
Fig.  4. 


481K 

470 
460 

450 

Q 
°o,440 

£3 

g  430 

& 

I'420 
$ 

410 
400 
390 

380 

( 

\ 

* 

N 

N 

x 

\ 

>w 

\ 

\ 

\ 

^ 

>  C 

>=* 

rs 

\ 

N 

\ 

> 

)        5        10       15       20,      25       30       35'      40       45      50       55       G( 
Seconds. 

FIG.  3. 

In  all  cases,  however  where  a  change  of  state  is  involved,  the 
solidification  point  is  marked  in  the  cooling  curve,*  by  means  of 
which  we  can  determine  its  position.  This  point  has  a  special 
meaning  in  complex  systems  of  mixtures  and  alloys  as  we  shall 
see  later. 

Polymorphy. 

Transitions  may  take  place  in  the  solid  state ;  for  example,  we 
know  tin  as  a  malleable  silver  white  metal,  but  a  remarkable 

*  For  the  methods  of  determining  cooling  curves,  see  Burgess,  Bulletin  Bureau 
of  Standards,  1908,  5,  199;  also  an  excellent  chapter  in  Deschs'  "Metallography," 
p.  123. 


12 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


change  is  sometimes  observed,  especially  in  cold  regions.  In  the 
mass  there  appear  here  and  there  spots  of  a  gray  color;  the  metal 
falls  to  a  brittle  product  which  occupies  a  much  greater  volume 
than  the  material  out  of  which  it  was  formed.  Due  to  this 
volume  change  there  forms  at  the  transition  places  "  Pustules  " 
that  fall  to  pieces  by  touching.  This  transformation  begins  at 
one  spot  and  travels  out  from  it  until  it  involves  the  entire 
vicinity  and  the  metal  breaks  to  a  brittle  powder.  Where  this 


Time 


FIG.  4. 


phenomenon  once  shows  itself  all  tin  is  in  danger.  If  a  grain  of 
the  transition  product  be  placed  on  the  intact  metal  the  transi- 
tion is  brought  on  there.  The  metal  is  said  to  be  "  sick;"  it 
becomes  the  tin  "  pest." 

This  phenomenon  was  first  observed  in  Russia,  where,  in  a 
military  magazine,  a  block  of  tin  was  found  fallen  entirely  to  a 
powder.  The  tin  pest  is  also  known  in  Germany;  for  example, 
the  eaves  on  the  Post  building  at  Rothenberg  are  infected  with  it. 


INTRODUCTION— PROPERTIES  OF  METALS  13 

A  number  of  chemists  have  investigated  the  tin  pest;  its  com- 
plete explanation  is  due  to  Schaum*  and  Cohen,  f 

The  chemical  analysis  of  the  brittle  powder,  to  which  the 
white  tin  falls,  shows  nothing  but  metallic  tin.  The  tin  pest  has 
thus  caused  no  chemical  change.  It  cannot  accordingly  be  con- 
sidered otherwise  than  a  new  allotropic  modification  of  tin, 
which,  because  of  its  gray  color  is  designated  as  gray  tin. 

The  above-named  investigators  have  now  established  the 
reciprocal  relations  of  the  two  forms  of  tin.  If  white  tin  be  pow- 
dered by  allowing  the  molten  metal  to  crystallize  with  strong 


FIG.  5.— Tin  Infected  with  the  Tin  "Pest." 

rubbing  in  a  mortar  and  this  powder  mixed  with  some  gray  tin 
and  placed  in  a  Dewar  vessel  for  several  days  at  the  tempera- 
ture of  solid  carbon  dioxide  and  ether,  the  entire  mass  changes 
to  gray  tin.  The  reverse  change  of  gray  tin  into  white  tin  can  be 
accomplished  by  warming  the  mass  on  a  water  bath. 

One  may  accordingly  prepare  at  will,  by  simple  temperature 
changes,  either  the  white  or  the  gray  modification.  The  process, 
as  we  have  seen,  is  entirely  reversible.  At  low  temperatures 
the  gray  is  the  stable  form,  at  high  temperatures  the  white. 
Between  these  temperatures  there  must  be  a  point  at  which  both 
*  Ann.  308,  30  (1899).  fZ.  Physik.  Chem.,  30,  601  (1899). 


14  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

forms  are  equally  stable  and  are  in  equilibrium  with  each  other. 
By  experiments  on  the  electro-motive  force  at  various  tempera- 
tures, of  a  galvanic  cell,  formed  from  the  two  modifications  of  tin 
as  electrodes,  in  a  stannous  chloride  solution  as  electrolyte,  it  is 
possible  to  determine  the  position  of  this  equilibrium.  At 
equilibrium  the  potential  difference  of  the  two  modifications  is 
nil.  At  high  temperatures  the  gray  modification  is  the  pos- 
itive pole  and  the  metastable  modification,  at  low  temperatures 
the  white.  The  equilibrium  temperature  is  about  20°.  Below 
this  point  a  transformation  occurs  to  the  gray,  above  it,  to  the 
white.  The  two  differ  also  in  their  crystalline  form,  the  gray 
having  a  columnar  structure. 

Analogy  of  Polymorphic  Transition  to  Melting. 

Such  a  multiplicity  of  forms  or  polymorphy  in  the  solid  state 
is  also  found  in  other  elements,  e.g.,  sulfur,  of  which  there  are 
rhombic  and  monoclinic  forms.  For  this  element  the  poly- 
morphic transition  and  its  peculiarities  have  been  carefully 
studied.  It  has  been  shown  that  an  extraordinary  similarity 
exists  between  the  process  of  polymorphic  transition  and  the 
process  of  melting  and  solidification.  The  equilibrium  point 
which  is  also  designated  as  a  transition  point,  can  be  compared 
in  many  ways  to  the  melting  point.  The  transition  of  one 
modification  into  the  other  is  accompanied  by  a  change  of  den- 
sity that  may,  in  some  cases,  be  very  considerable.  In  the  transi- 
tion of  gray  to  white  tin  it  increases  from  5.85  to  7.30.  As  with 
melting  the  transition  is  accompanied  by  an  absorption  of  heat. 
The  so-called  transition  heat  is  however  ordinarily  less  than  the 
heat  of  fusion.  The  two  processes  differ  in  only  one  respect, 
namely,  the  low  temperature  form  can  frequently  be  observed 
above  the  transition  point,  while  it  is  not  possible  to  heat  a  solid 
substance  above  its  melting  point  without  its  going  over  to  a 
liquid. 

Determination  of  the  Transition  Point. 

The  phenomena  of  volume  change  and  transition  heat  can 
both  be  used  to  ascertain  the  position  of  the  transition  point. 


INTRODUCTION— PROPERTIES  OF  METALS 


15 


It  is  of  special  importance,  for  example,  with  iron  which  has  at 
least  three  polymorphic  modifications  sharply  separated  from 
each  other  by  two  transition  points.  We  have  the  three  forms 
a  iron  or  ferrite,  and  |8  and  7  iron.  The  transition  point  a.  — >  /? 
lies  at  768°  C.,  the  second  0->  7  at  898-909°  C* 

The  cooling  curve  of  iron  shows  slight  arrests  at  these  tem- 
peratures which  are  brought  about  in  precisely  the  same  way  as 


Temperature  °C. 

\ 

1530° 

Solidi 

ficatk 

n  Poii 

It 

\ 

S 

VIr 

on 

i 
i 

\ 

/ 
/ 
/ 
/ 

\ 

/' 
/ 

/ 

AC> 

/ 

j/905 

898°  / 

^  3 

800 

/ 
/ 

s 

\r 

on 

/ 

Ac— 

r\ 

768°  i 
\ 

Vr2 

// 

\ 

Oi  Iror 
\ 

/ 
/ 

X 

X 

10   20   30   40   50   60   7Q 
Minutes 

FIG.  6. 


90   100 


the  points  of  arrest  due  to  the  solidification  of  a  molten  liquid. 
Fig.  6  shows  a  cooling  curve  for  pure  iron. 

The  volume  change  of  the  transition  process  is  the  cause  of 
the  re-expansion  of  iron  strips  on  cooling  which  has  been  observed 
by  Kinder.  The  length  change  in  a  strip  90  meters  long  is 
about  200  mm.  The  change  of  7  into  /3  iron  is  accompanied  by 
a  volume  increase,  the  7  iron  being  denser  than  the  /3,  but  the 


*  Burgess  and  Crowe,  Bur.  Standards  Bull.,  10,  317  (1913).    This  paper  also 
contains  an  excellent  bibliography  on  the  allotropy  of  iron. 


16 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


change  of  the  latter  into  the  a  modification  gives  again  a  denser 
product  and  a  contraction  of  the  piece.* 

The  a  iron  possesses  the  property  of  magnetism  which  the 
other  forms  do  not.  This  can  be  easily  shown  by  a  simple 
experiment  (Fig.  7).  If  a  small  piece  of  wrought  iron  be  sus- 
pended on  a  wire  before  a  magnet  it  will  naturally  be  drawn  to 
the  magnet,  but  if  it  be  heated  to  a  white  heat,  the  action  of  the 


FIG.  7. 


magnet  ceases.  The  attraction  occurs  again  on  cooling  when  the 
temperature  has  sunk  below  768°  C.  the  transition  point  for  the 
reaction  j8  — >  a.  Accordingly  if  the  /3  or  the  7  were  the  stable 
form  at  ordinary  temperatures  our  entire  electrical  industry  in 

*The  volume  changes  of  the  iron  transitions  have  been  studied  by  Charpy 
and  Grenet,  Bull.  Soc.  d'Encouragement,  104,  464  (1892);  Broniewski,  Compt. 
rend.,  p.  1983  (1913);  Rosenhain  and  Humphrey,  Proc.  Roy.  Soc.,  83,  200  (1909); 
Le  Chatelier,  Compt.  rend.,  129,  279  (1899). 


INTRODUCTION— PROPERTIES  OF  METALS 


17 


the  form  in  which  we  now  possess  it  would  be  impossible.  Dyn- 
amo machines  and  electric  motors  could  not  be  built  if  we  did 
not  possess  a  material  so  strongly  magnetic  as  a  iron. 

We  also  encounter  polymorphy  in  other  metals.  The  fol- 
lowing table  from  Guertler*  gives  the  transition  points  of  the  ele- 
ments so  far  as  they  have  been  determined.! 


Metal. 

Transition  Points,  °C. 

Iron    . 

808  768 

Cobalt  

I  I^O 

Nickel  .  .  . 

7  2O 

Zinc 

l6<D 

Aluminum  

«?6o 

Thallium 

226 

Tin  

170,  18 

Explosive  Antimony. 

The  changes  which  a  second  form  of  antimony  undergoes  are 
very  remarkable  and  represent  another  kind  of  dimorphy. 
Antimony  which  has  been  precipitated  electrolytically  from  an 
antimony  chloride  solution  on  a  copper  wire  as  cathode,  pos- 
sesses a  smaller  density  than  the  ordinary  form,  namely  5.78 
against  6.52  and  is  also  different  in  appearance,  being  darker 
and  more  lustrous.  By  touching  it  with  a  point  an  explosion 
follows,  the  metal  becomes  strongly  heated  and  throws  off  white 
vapors.  The  vapor  evolution  has  nothing  to  do  with  the  transi- 
tion process,  the  vapors  being  chloride  which  the  metal  has  ad- 
sorbed. The  ordinary  modification  under  similar  conditions 
also  has  a  tendency  to  take  up  the  chloride.  If  a  particle  loos- 
ened from  the  electrode  be  rubbed  briskly  in  a  mortar  a  detona- 
tion ensues  with  the  evolution  of  light  and  heat.  The  modifica- 
tion transforms  explosively  and  hence  is  known  as  explosive  anti- 

*  "  Metallographie,"  Vol.  i  p.  112. 

t  According  to  Getman,  J.  Am.  Chem.  Soc.,  39,  1806  (1917),  cadmium  exists 
in  two  allotropic  modifications,  with  a  transition  at  37.5°  C.  According  to  Cohen, 
J.  Am.  Chem.  Soc.,  40,  1149  (1918)  there  are  three  modifications  of  cadmium, 
one  of  which  is  metastable.  The  transition  point  of  the  stable  forms  is  given 
as  approximately  60°  C. 


18  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

mony.  The  explosion  also  occurs  if  the  metal  is  heated  to  200°  C. 
Cohen  *  and  his  students  who  have  investigated  this  remarkable 
substance  give  an  experiment  which  is  designed  to  show  the 
transition  heat.  If  to  10  gm.  of  the  explosive  material  con- 
tained in  a  thick-walled  tube  5  cc.  of  ether  be  added  and  the  tube 
stoppered,  by  shaking  the  explosive 
transformation  takes  place  and  the  heat 
liberated  vaporizes  a  considerable  quan- 
tity of  ether  and  the  stopper  is  blown 
from  the  tube  with  violence.  The 
appearance  of  explosive  antimony  is 
shown  in  Fig.  8.  The  transition  heat  is 
20-21  small  calories  per  gram,  the  pro- 
duct of  the  transition  being  ordinary 
antimony. 

Enantiotropy  and  Monotropy. 

While  with  tin  and  iron  we  can 
make  the  transition  go  in  either  direc- 
tion by  simple  change  of  temperature, 
it  is  not  possible  to  reverse  the  transi- 
tion of  antimony  in  this  way.  The 
reaction  proceeds  always  in  the  direction 
of  explosive  to  ordinary  antimony  and 
possesses  at  no  temperature  an  equili- 
brium or  transition  point.  The  explosive 
form  is  at  all  temperatures  metastable. 
Lehmann  has  suggested  a  special  name 
for  each  of  these  two  cases  of  dimorphy. 
He  designates  the  first  as  enantiotropy, 

the  second  monotropy. 
no.  8.-Rod  of  Explosive  ^  ^  ^  ^  ^         ^ 


Antimony.  ...  .  .  . 

as  can  be  readily  understood  by  con- 

sulting the  vapor  pressure  diagram.     A  vapor  pressure  curve  can 
be  determined  for  every  polymorphic  modification  and  equili- 

*Z.  Physik.  Chem.,  47,  i  (1904);   50,  291  (1904);  52,  129  (1905);   Z.  Elek- 
trochem.,  11,  787  (1905). 


INTRODUCTION— PROPERTIES  OF  METALS 


19 


brium  between  two  forms  can  only  exist  then  where  two  vapor- 
pressure  curves  intersect.  At  this  point  the  two  vapor-pressures 
are  equal  and  there  is  no  tendency  to  change  in  either  direction. 
If  a  substance  is  heated  sufficiently  high,  it  finally  melts,  and 
by  super-heating  a  melting  point  can  be  established  for  meta- 
stable  modifications.  With  sulfur,  for  example,  melting  points 
have  been  determined  for  the  monoclinic  as  well  as  for  the  rhom- 
bic variety.  If  we  wish  to  determine  the  relative  position  of  the 


Monotropy 


Temp. 


Enantiotropy 


Temp. 


FIG.  9. 


transition  point  and  the  melting  point  we  must  take  into  account 
the  vapor-pressure  curves  of  the  two  solids,  as  well  as  that  of  the 
liquid  melt.  The  curve  for  the  liquid  can  cut  the  curves  for 
the  solid  modifications  in  two  ways,  as  shown  in  Fig.  9,  either 
above  or  below  the  transition  point.  The  temperature  of  the 
intersection  is  the  melting  temperature.  It  can  be  further 
concluded  from  the  diagram  that  the  melting  point  of  the  stable 
form  with  the  lower  vapor  pressure  lies  at  a  higher  temperature 
than  that  of  the  metastable.  If  the  melting  point  is  above  the 


20  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

transition  point  we  have  the  case  of  enantiotropy.  If  we  neat 
the  low  temperature  modification  gradually  we  come  first  to 
the  transition  point,  by  overstepping  this  the  second  form  appears 
and  finally  we  reach  the  melting  point.  If  the  melting  point  is 
below  the  transition  point  we  have  the  case  of  monotropy.  We 
can  therefore  not  reach  the  transition  point  since  both  forms  go 
over  previously  to  the  liquid  state.  One  form  must  therefore 
always  be  metastable  and  the  transition  can  proceed  in  only  one 
direction. 

Crystal  Growth.   . 

A  structure  change  can  also  be  observed  in  iron  which  is  pure 
or  very  poor  in  carbon,  if  it  be  heated  for  a  long  time  at  a  tem- 
perature of  650-700°  C.  At  this  temperature  the  principal  con- 
stituent is  a  iron  or  ferrite.  Under  the  microscope  it  is  recog- 
nized by  the  polygonal  form  of  the  small  crystals  (see  chapter  3). 
Under  ordinary  conditions  a  piece  of  iron  which  was  heated  to  a 
high  temperature  and  then  cooled,  shows  a  finely  crystalline 
structure.  By  the  above-mentioned  thermal  treatment,  how- 
ever, a  large  crystalline  structure  is  obtained,  which  influences 
the  mechanical  properties  in  an  unfavorable  way.  The  mate- 
rial is  harder  and  its  solidity  is  diminished.  The  large  crystals 
break  more  easily  than  the  small  crystals.  (Kinder).  This 
change  in  structure  and  properties  is  not  due  to  a  change  in 
modification,  but  to  an  enlarging  of  the  single  crystals  by  a  grad- 
ual recrystallization. 

It  is  a  frequently  observed  fact,  that  at  constant  temperature 
large  crystals  grow  while  the  smaller  ones  are  consumed.  If  for 
example,  a  crystallized  salt,  say  saltpeter,  is  allowed  to  stand 
under  the  mother  liquor  at  room  temperature,  the  same  relations 
are  shown,  the  structure  of  the  solid  phase  becomes  largely  crys- 
talline. These  facts  remind  one  of  the  growth  of  a  large  liquid 
drop  which  is  found  in  the  vicinity  of  small  drops.  According 
to  W.  Thomson  small  drops  have  a  somewhat  higher  vapor  pres- 
sure than  large  drops.  Accordingly  if  the  temperature  is  con- 
stant, a  distillation  takes  place  from  the  small  drops  to  the  larger 
ones  and  there  is  consequently  a  growth  of  the  latter.  Small 


INTRODUCTION— PROPERTIES  OF  METALS  21 

crystals  possess  a  larger  vapor  tension  and  a  higher  solubility 
than  large  ones.  The  large  crystals  are  accordingly  more  stable 
and  are  formed  spontaneously  from  the  smaller  ones. 

The  enlargement  of  the  structure  by  long-continued  heating 
of  iron  is,  therefore,  a  special  case  of  this  entirely  general  rule. 
The  process  goes  on  as  do  all  reactions,  faster  as  the  temperature 
increases.  The  upper  temperature  limit  is  formed  here  by  the 
transition  point  a  — >  0  iron.  If  this  is  exceeded  the  large  a  crys- 
tals fall  to  pieces  to  small  0  crystals  so  that  the  structure  again 
becomes  finely  crystalline. 

Electrical  and  Optical  Properties  of  Metals :  Conductivity. 

The  previously  considered  phenomena  are  of  an  entirely 
general  nature  and  are  in  no  way  peculiar  to  the  metallic  state. 
There  is  however  a  whole  series  of  properties  which  are  charac- 
teristic of  the  metals.  The  lustrous  appearance  and  the  ability 
to  conduct  heat  and  electricity  distinguish  the  metals  from  all 
other  substances.  These  properties  of  the  elementary  metals 
appear  in  a  weak  degree,  in  certain  metallic  compounds  espe- 
cially the  sulfides.  For  an  insight  into  the  nature  of  metals  we 
are  indebted  to  the  views  of  physicists  on  the  nature  of  elec- 
tricity, the  so-called  electron  theory. 

Electrical  conductivity  is  not  characteristic  of  the  metals 
alone  but  solutions  of  acids,  bases  and  salts,  as  well  as  molten 
salts  and  certain  solid  oxides,  such  as  constitute  the  Nernst 
glower,  conduct  the  electric  current.  A  theory  has  been  devel- 
oped by  which  we  are  able  to  account  for  each  of  these  things. 

The  conduction  of  electricity  by  acids,  bases  and  salts  dis- 
solved in  water  has  led  to  the  theory  that  these  substances  are 
dissociated  in  solution  into  electrically  charged  split  molecules. 
The  products  of  the  dissociation  are  ions,  that  is,  electrically 
charged  atoms  or  atom  complexes.  It  follows  from  the  dis- 
sociation process  that  there  are  as  many  positive  as  negative 
charges,  the  number  of  charges  which  an  ion  possesses  expressing 
its  valence.  The  positively  charged  ions  migrate  to  the  cathode 
and  are  called  cathions,  while  the  negative  anions  go  to  the  anode. 

If  a  solution  of  an  electrolyte  be  brought  between  two  elec- 


22  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

trodes  and  the  circuit  closed,  the  electrodes  become  charged,  one 
with  positive  and  the  other  with  negative  electricity.  Under 
the  influence  of  the  charges,  motion  of  the  ions  to  the  oppositely 
charged  electrode  results.  On  its  arrival  the  electrically  charged 
particle  gives  up  its  charge  and  becomes  electrically  neutral. 
From  the  silver  ion  metallic  silver  results,  from  the  chlorine  ion, 
chlorine  gas,  etc.  The  passage  of  the  current,  therefore,  involves 
chemical  changes,  the  electrolyte  being  decomposed  at  the  elec- 
trodes. 

The  transport  of  electricity  through  the  liquid  is  accomplished 
by  ions.  The  displacement  of  the  electrical  charges  bound  to  the 
material  particles  is  greater,  the  stronger  the  current.  The  con- 
ductivity is  greater  the  greater  the  concentration  of  ions  and  the 
smaller  the  frictional  resistance  which  their  motion  inside  the 
liquid  encounters.  The  movement  of  the  cathion  and  the  anion 
in  the  same  electrolyte  is  generally  unequal,  and  as  a  result  of 
such  migration  differences,  concentration  displacements  occur 
in  the  vicinity  of  the  electrodes.  There  is  an  increase  in  the 
concentration  at  the  electrode,  of  the  ion  which  moves  in  that 
direction  and  a  decrease  in  the  concentration  of  the  other  ion. 

Faraday's  Law. 

The  amount  of  the  electrode  discharges  and  of  substance  pre- 
cipitated depend  on  the  current  strength.  Faraday  has  shown 
that  by  passing  the  same  current  through  different  cells,  the 
amounts  of  different  substances  precipitated  are  in  the  ratio  of 
their  equivalent  weights.  Equivalent  amounts  of  ions  carry 
equal  amounts  of  electricity.  The  gram  equivalent  carries  96,540 
coulombs,  a  current  of  i  ampere  must  flow  96,540  seconds  or 
26.8  hours  to  precipitate  108  gm.  of  silver,  i.oi  gm.  of  hydrogen 
or  31.8  gm.  of  copper. 

Metallic  and  Gaseous  Conduction. 

In  the  conduction  of  electricity  through  metals  there  is  no 
displacement  by  the  current.  The  conductor  remains  abso- 
lutely unaltered.  In  this  circumstance  the  metallic  conductors 
do  not  stand  alone,  a  similar  relation  existing  in  the  conduction 


INTRODUCTION— PROPERTIES  OF  METALS  23 

of  current  by  dilute  gases  under  the  action  of  cathode  rays  and 
/8  Bequerel  rays  sent  out  by  radioactive  substances.  These  rays 
consist  of  negatively  charged  electrical  particles  streaming  with 
great  velocity,  which  can  be  deflected  from  their  path  by  electro- 
static and  electro-magnetic  influences.  From  the  magnitude 
of  the  electro-static  and  electro-magnetic  deflection  both  the 
velocity  of  the  charged  particles  and  their  mass,  which  is  the 
unit  of  electrical  transportation,  can  be  calculated. 

Electron  Theory. 

For  the  mass  we  get  a  very  small  value.  One  gram  of  sub- 
stance will  transport  i  .88  X  io8  coulombs.  One  gram  of  hydrogen, 
the  lightest  of  all  known  substances,  carries  in  the  ionic  state, 
96,540  coulombs.  It  is  calculated  therefrom  that  the  equivalent 
weight  of  the  ray  substance  is  0.000513.  In  grams  this  is  the 
mass  of  cathode  and  /3  Bequerel  rays  which  will  be  associated 
with  96,540  coulombs.  Other  phenomena  have  also  been  used  to 
determine  this  magnitude  and  give  about  the  same  value.  These 
carriers  of  negative  charges  which  are  in  round  numbers  2000 
times  as  light  as  hydrogen  particles,  are  now  generally  desig- 
nated as  electrons.  It  remains  to  be  pointed  out  that  such  small 
carriers  of  positive  charges  are  not  known. 

The  electrons  also  cause  current  conduction  inside  the  metals 
according  to  the  new  view.  The  electron  theory  of  metals  that 
has  been  evolved  by  J.  J.  Thomson  and  P.  Drude  *  supposes 
that  within  the  metal  a  dissociation  is  undergone  into  positive 
metal  ions  and  negative  electrons,  and  that  the  negative  par- 
ticles, under  the  influence  of  the  potential  difference  attached  to 
the  metal,  fall  to  streaming  in  the  same  way  as  the  electrolytic 
ions  in  an  electrolytic  cell. 

Law  of  Wiedemann  and  Franz. 

Wiedemann  and  Franz  have  noted  a  remarkable  parallelism 
between  the  electrical  and  heat  conductivity  of  metals.  The 
good  conductors  of  electricity  being  also  good  conductors  of 

*  Ann.  Physik.,  1,  566  (1900);  8,369(1900);   7,  687   (1902);   14,936   (1904); 
Physik.  Z.,  1,  161  (1900). 


24 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


heat  and  if  the  ratio  of  the  heat  conductivity  K  to  the  elec- 
trical conductivity  a  is  calculated  it  is  shown  that  for  the  same 
temperature  this  ratio  possesses  a  large  value  which  varies  within 
narrow  limits.  This  fact  is  shown  in  the  following  table  from  the 
experiments  of  Jaeger  and  Diesselhorst:* 


Metal. 

-  for  18°. 
ff 

*  for  100°. 

_K_  .     K 
al8  '  trioo* 

Aluminum  

636 

844 

I.  12 

Copper.  .  . 

665 

862 

.  7Q 

Silver  

686 

881 

28 

Gold 

727 

Q2<C 

27 

Nickel 

600 

006 

•3Q 

Zinc  

672 

867 

.  2O 

Cadmium  ....         .    . 

706 

OCX 

28 

Lead 

7ir 

Q7r 

31 

Tin  

7?c 

Q2<; 

.26 

Platinum  
Iron  

753 
802 

1013 
1061 

•35 

32 

Bismuth 

063 

1067 

12 

A  theory  of  these  remarkable  facts  has  been  brought  forward 
by  Drude  who  explains  the  parallelism  between  the  two  con- 
ductivities by  the  assumption  that  the  electrons  cause  both  the 
heat  and  electrical  vibrations  within  the  metals.  To  get  a  rep- 
resentation of  the  mechanism  of  heat  conductivity  by  means  of 
the  electrons,  he  made  the  bold  but  successful  assumption 
that  the  electrons  in  the  metal  followed  the  same  diffusion  law 
as  the  molecules  of  a  gas. 

The  kinetic  theory  of  gases  starts  from  the  assumption  that 
the  gas  molecules  are  in  a  rapid,  entirely  irregular  motion  and 
that  the  average  kinetic  energy  of  the  molecules  of  a  gas  depends 
on  and  is  proportional  to  the  absolute  temperature.  And, 
further,  that  the  temperature  coefficient  of  kinetic  energy  pos- 
sesses the  same  value  for  all  gases.  For  the  electrons  inside  the 
metal  Drude  postulated  that  a  similar  irregular  motion  also  took 
place  and  that  they  possessed  a  kinetic  energy  which  is  equal 


*  Wissenschaftliche  Abhandlungen  der  Phys.  Techn.  Reichsanstalt,  3,  269. 


INTRODUCTION—  PROPERTIES   OF  METALS  25 

to  that  of  the  gas  molecules  at  the  same  temperature.  This 
assumption  requires  of  the  very  small  mass  of  the  electrons,  a 
very  great  velocity  compared  to  the  gas  molecules.  If  the 
mass  of  the  electron  is  placed  at  about  one  four-thousandth  of 
that  of  the  hydrogen  molecule  Eb,  the  velocity  of  electrons  of 
the  same  kinetic  energy  (JMV2)  must  be  more  than  60  times  as 
great  as  that  of  the  hydrogen  molecule,  which  is  recognized  as  the 
fastest  of  material  molecules. 

The  higher  the  temperature  the  greater  the  velocity,  and  the 
greater  the  path  which  the  moving  particle  describes  before  col- 
lision. The  so-called  mean  free  path  increases. 

The  heat  interchange  in  an  air  column,  the  upper  part  of 
which  is  warmer  than  the  lower,  takes  place  in  the  following  way: 
particles  from  the  warmer  layers  having  a  higher  velocity, 
advance  to  the  colder  and  the  ascension  of  the  particles  exerts 
an  active  force,  on  the  particles  above,  whose  energy  is  increased 
by  the  collisions.  In  an  entirely  similar  way  Drude  explains 
the  mechanism  of  the  heat  conduction  in  metals  which  have  one 
end  hot  and  the  other  end  cold. 

By  the  mathematical  formulation  of  these  views  Drude 
arrived  at  the  equation,  K  =  \a  Nlu  in  which  the  heat  conductivity 
K  is  set  equal  to  the  electron  number  N  multiplied  by  the  mean 
velocity  of  the  electrons  «,  the  mean  free  path  between  two  col- 
lisions /  and  the  temperature  coefficient  of  the  active  force  a 
that  possesses  the  same  value  for  gas  molecules  and  electrons. 

The  electrical  conductivity  is  likewise  influenced  by  the 
irregular  movements  of  free  ions.  It  may  not  be  assumed  that 
the  applied  electro-motive  force  transforms  the  irregular  move- 
ment into  a  regular  one.  It  merely  causes  one  direction  of  move- 
ment of  the  electrons  to  be  somewhat  favored  so  that  a  streaming 
in  that  direction  results.  Collisions  and  direction  changes  of 
single  ions  do  not  thereby  cease.  Drude  derived  for  the  elec- 
trical conductivity  the  formula 


where  e  is  the  charge  on  one  electron  equivalent. 


26  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

By  the  division  of  the  two  equations  in  a  way  to  eliminate  the 
expression  Nlu  it  follows  that 


The  only  variable  on  the  right-hand  side  of  this  expression  is 
the  temperature,  a  and  e  are  characteristic  constants  for  the 
individual  metals.  This  relation  is  at  constant  temperature, 
independent  of  the  nature  of  the  metal.  If  we  know  a  and  e 
the  ratio  of  the  two  conductivities  can  be  calculated  for  a  given 


*   r       K 


temperature.     The  calculation  of  Reinganum  *  for  —  at  18° 

(7 

gives  the  theoretical  value  709.9  •  io~8. 

The  law  of  Wiedemann  and  Franz  that  for  all  metals  the 
relation  between  heat  and  electrical  conductivity  is  nearly  con- 
stant can  accordingly  be  derived  from  the  theory. 

The  equation  admits  of  a  still  further  test  of  the  basis  on 
which  it  is  built.  The  conductivity  ratio  must  be  proportional 
to  the  absolute  temperature  and  must  be  expressed  by 


(f). '(?).- 


If  it  be  observed  at  temperatures  of  18°  (T  =  2gi)    and  100° 
the  ratio  of  the  conductivity  quotients  must  have  a 


value  ^-^  =  1.28,  if  the  actual  value  be  now  compared  with  the 
291 

theoretically  calculated  value,  there  is  seen  a  good  agreement. 
(See  table  on  page  24.) 

The  hypothesis  of  the  electron  theory  of  metals  originated 
by  Drude  has  accordingly  proved  itself  a  valuable  research  prin- 
ciple. 

The  magnitude  of  the  electrical  conductivity  of  a  metal 
depends  like  that  of  a  solution,  on  two  factors,  the  number  of 
conduction  electrons  present  and  the  friction  which  these  en- 
counter in  their  motion  through  the  medium.  It  is  of  special 

*  Ann.  Physik.,  2,  398  (1900). 


INTRODUCTION— PROPERTIES  OF  METALS  27 

interest  to  know  the  electron  number  or  electron  concentration 
in  a  single  metal.  The  relations  which  we  can  derive  from  the 
equation  connecting  the  electron  number  with  the  mobility  will 
not  however  accomplish  this  calculation.  It  is  necessary  for 
this  purpose  to  have  a  knowledge  of  further  relations  between 
these  magnitudes.  This  leads  us  to  the  optical  properties  of 
metals. 

Optical  Properties  of  Metals :  Light  Reflection  and  Absorption. 

The  metals  are  characterized  optically  by  their  special  kind 
of  luster,  the  metallic  luster.  Other  substances  have  also,  under 
certain  conditions,  a  luster  similar  to  the  metals.  The  silver 
luster  of  a  water  surface  in  the  moonlight  is  known  to  all.  An 


FIG.  10. — Fuchsin  Paper. 

air  blast  under  water  has,  if  viewed  in  the  proper  direction,  the 
appearance  of  mercury;  the  same  is  true  of  the  total  reflection 
prism.  In  all  these  cases  the  cause  of  the  luster  is  the  complete 
reflection  of  incident  light.  Certain  organic  dyes  also  show  a 
strong  generally  colored  surface  luster.  A  piece  of  indigo  is 
lustrous  like  copper,  especially  if  it  has  a  somewhat  polished  sur- 
face. The  crystals  of  Fuchsin  and  of  methyl  violet  appear 
greenish  gold.  These  lusters  are  also  connected  with  strong 
reflection  of  light.  (Fig.  10.) 

If  we  dissolve  these  dyes,  and  view  the  sun's  spectrum  through 
a  layer  of  this  solution,  we  observe  a  strong  absorption  band. 
The  fuchsin  solution,  for  example,  allows  red  and  yellow  to  pass 
through  but  not  green,  this  is  absorbed  and,  in  place  of  the  green 


28 


THE  PHYSICAL   CHEMISTRY  OF  THE  METALS 


part  of  the  spectrum,  there  is  seen  a  black  band.  The  green 
light  is  now  accordingly  reflected  from  the  solution.  It  is  a 
general  fact  that  a  substance  reflects  the  kind  of  light  for  which  it 
possesses  an  absorption.  We  see  then  that  light  absorption  and 
reflection  as  well  as  the  luster  are  all  connected. 

Metallic  Luster. 

Metallic  luster  is  no  exception  to  this  rule.  In  reference  to 
light  absorption,  the  metals  exceed  all  other  substances.  Only 
the  thinnest  metal  leaves  are  transparent.  Thin  gold  leaves 
allow  blue  light  through  while  yellow  light  is  strongly  absorbed 
by  them  and  is  accordingly  especially  strongly  reflected.  For 
very  thin  metal  layers  precipitated  on  glass,  Kundt  has  mea- 
sured refraction  and  absorption  for  different  kinds  of  light,  but 
this  procedure  presents  very  great  experimental  difficulties.  It 
is  not  easy  to  obtain  uniform,  adherent  metal  coats  a  few  thou- 
sandths of  a  millimeter  thick.  Further,  the  errors  of  observa- 
tion in  the  measurement  are  relatively  large,  due  to  the  very 
sharp  prism  which  must  be  used. 

Therefore  Drude  made  use  of  another  phenomenon  to  deter- 
mine the  optical  constants  of  metals,  their  index  of  refraction  n 
and  their  absorption  coefficient  k.  The  metals  reflect  incident 
linear  polarized  light  as  elliptically  polarized.  From  the  mag- 
nitude of  this  displacement  there  is  given  the  sought  values  and 
the  reflection  constant.  The  following  table  contains  the  con- 
stants measured  by  Drude  *  and  the  reflection  constant  R  for  a 
number  of  different  metals  for  yellow  sodium  light. 


Metal. 

R  Per  Cent. 

2V.  K. 

2V. 

Silver 

O"\    3 

1  6? 

o  18 

Gold          

85.1 

2.82 

0.37 

Platinum       

70.  1 

4.  26 

2.06 

CoDoer 

72    2 

2.62 

0.64 

Steel 

eg   C 

2     4O 

2.41 

Sodium 

00   7 

2    6l 

O  OCX 

78.4 

4.96 

1.73 

*  Ann.  Physik.,  14,  936  (1904). 


INTRODUCTION— PROPERTIES  OF  METALS  29 

The  absorption  and  therewith  the  reflection  constant  is  very 
different  for  different  kinds  of  light  and  different  wave  lengths. 
With  copper  it  is  especially  large  for  red,  with  gold  for  yellow 
light. 

The  light  absorption  is  brought  about  in  a  small  degree,. espe- 
cially in  the  colored  metals,  by  a  similar  cause  as  with  the  aniline 
dyes.  In  them  the  characteristic  vibrations  reside  in  the  mole- 
cules, but  the  determined  equilibrium  position  of  the  vibrating 
electrons  agrees  with  the  period  of  the  absorbed  light.  It  can 
be  treated  accordingly  as  an  optical  resonance  phenomenon. 
With  gold,  copper,  silver  and  magnesium,  the  optical  relations 
are  determined  by  such  a  restricting  influence,  not  by  freely 
moving  electrons. 

The  freely  moving  conduction  electrons  possess  the  greatest 
influence  on  the  optical  properties  of  metals.  To  understand 
this,  we  must  call  to  mind  that,  according  to  the  electro-mag- 
netic theory  of  light,  a  light  wave  is  no  other  than  a  train  of 
electrical  waves,  that  is,  a  train  of  periodic  changes  of  electrical 
force.  The  action  of  this  periodic  electrical  force  calls  forth  a 
displacement  of  free  electrons  in  the  conductor,  in  the  same  way 
as  a  constantly  acting  electrical  force.  The  metals  are,  accord- 
ingly, good  conductors  for  the  electrical  waves,  as  well  as  for  the 
constant  current. 

The  quick  propagation  of  the  light  waves  in  a  medium  is, 
therefore,  closely  connected  with  strong  light  absorption.  The 
velocity  of  the  wave  displacement  inside  of  a  metal  is  now,  as  the 
electrical  conduction,  dependent  on  the  number  of  electrons, 
and  on  the  frictional  resistance,  and,  therefore,  the  optical  con- 
stants of  the  metals  and  their  reflection  constant  must  stand  in 
close  relation  to  both  of  these  factors.  There  must  be  a  parallel- 
ism between  the  reflection  constant  of  a  metal  and  its  electrical 
conductivity.  In  fact,  the  investigations  of  Hagen  and  Rubens  * 
on  the  reflection  of  long  infra-red  light  waves  yielded  so  exact  a 
parallelism  that  it  was  directly  possible  to  calculate  the  reflec- 
tion constant  for  these  waves  from  the  conductivity.  The 
agreement  decreases  with  decreasing  wave  length. 
*  Ann.  Physik.,  11,  873  (1903). 


30 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


Electron  Concentration  of  Metals. 

Schuster  *  has  deduced,  by  the  use  of  the  Drude  dispersion 
theory  of  metals,  a  relation  between  these  optical  properties,  the 
electron  number,  and  the  frictional  resistance.  If  we  combine 
this  with  the  above-mentioned  relations  which  connect  con- 
ductivity, electron  number  and  frictional  resistance,  we  have  a 
means  of  calculating  the  unknown  magnitudes,  electron  number 
and  frictional  resistance.  Unfortunately  this  way  is  not  feasible 
since  the  resistance  which  the  electrons  encounter,  in  streaming, 
is  not  proportional  to  the  quickness  of  oscillation.  The  friction  is, 
in  a  large  sense,  dependent  on  the  vibration  number,  which,  in  turn, 
depends  on  the  wave  length;  but  only  for  very  long  wave  lengths 
is  an  equality  between  it  and  the  friction  of  streaming  permissible. 

Now,  Drude  has  succeeded,  by  a  simplifying  assumption,  in 
deriving  a  relation  between  the  optical  constants,  index  of  refrac- 
tion and  coefficient  of  absorption,  and  the  electron  number. 
In  this  the  frictional  resistance  is  neglected.  With  the  help  of 
this  relation  and  his  observations  for  the  wave  length  X  =  o.  598^ 
(Yellow  Na  light) ,  he  calculated  the  following  electron  numbers 
for  the  different  metals. 


Metal. 

Electron  Number 
per  Atom. 
P 

Atomic  Volume. 
A 
~d 

Electron  Concen- 
tration per  Cubic 
Centimeter. 

»-4 

CoDoer 

o  47 

7  * 

0.066 

Gold  

o.  72 

IO.  2 

0.070 

Silver  

i.  06 

IO.3 

o.  103 

Nickel  

I    OO 

6.7 

o.  149 

Cobalt 

I    41? 

6.7 

o.  216 

Iron  

I.7O 

7-  2 

0.236 

Magnesium  
Platinum 

I.Q2 
2   OO 

13-3 

9.  1 

0.144 

O    22O. 

Aluminum 

2    20 

IO.  I 

o.  227 

Cadmium  

2.  50 

13.0 

0.192 

Zinc 

2    83 

0    "? 

o.  298 

Lead      

3    2O 

18.2 

o.  179 

Mercuiry 

•2      -2Q 

14.8 

0.228 

Bismuth 

3     66 

21  .  I 

o.  174 

Tin 

2      TI 

16  2 

o.  230 

Antimony 

7    ^4. 

17    0 

0.420 

Phil.  Mag.,  (6),  7,  151  (1904). 


INTRODUCTION— PROPERTIES  OF  METALS 


31 


One  can  come  to  a  conception  of  the  high  electron  concentra- 
tion, if  it  is  made  clear  how  many  electrons  are  contained  in  a 
liter,  by  expressing  how  many  times  normal  the  electron  solution 
is  with  relation  to  electrons.  If  we  arrange  the  metals  according 
to  the  magnitude  of  this  number,  we  obtain  the  following  table: 


Metal. 

Normality 
of  Electrons, 
Times. 

Thermo- 
electric 
Force,  ag.  Pt. 

Metal. 

Normality, 
Times. 

Thermo- 
Force. 

CoDDer 

66 

-|-o  72 

Cobalt  

216 

—  o  80 

Gold     

70 

+O.  72 

Platinum  

2  2O 

Silver  
Magnesium.  .  .  . 
Nickel  

103 
144 
140 

+0.71 
+0.42 
-1.62 

Aluminum  .... 
Mercury  
Tin  

227 
228 

2^O 

+0.40 
+0.72 
-l-O.4t; 

Bismuth 

—  *  So 

Iron  

236 

-1-1    ACT 

Lead  
Cadmium  

179 
192 

TO.  44 
+0.85 

Zinc  
Antimony  .... 

298 
42O 

+  0-75 
+  5-II 

The  electron  concentration  of  metals,  is,  accordingly,  very 
considerable  and  their  large  conductivity  is  thereby  easily 
understood. 

Temperature  Coefficient  of  Conductivity. 

With  increasing  temperature  the  conductivity  decreases,  and 
indeed  by  a  temperature  raise  of  i°  the  decrease  amounts  to 
practically  2~7~3.  By  what  is  this  change  caused?  It  can  be 
explained  by  a  lowering  of  the  electron  concentration  or  by  an 
increase  in  frictional  resistance  with  rising  temperature.  The 
investigation  of  optical  properties  serves  to  decide  this  question. 
The  optical  constants  which  have  been  determined  by  Rosch- 
destwensky,  a  pupil  of  Drude,  at  the  temperatures  of  solid  carbon 
dioxide  and  ether  and  of  liquid  air,  show  small  differences  from 
those  at  ordinary  temperatures.  According  to  Zeeman,  the 
optical  properties  of  a  metal  change  only  slightly  by  heating  to 
800°.  The  electron  concentration  is  accordingly  nearly  constant 
throughout  a  temperature  interval  of  about  1000°. 

The  strong  change  of  the  metallic  conductivity  is  then 
referred  to  the  frictional  resistance.  The  increase  of  resistance 
with  rising  temperature  is  easily  understood,  if  we  recall,  that 
also  the  internal  friction  (viscosity)  of  a  gas  increases  with  in- 


32  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

creasing  temperature.  Now  from  the  kinetic  theory  of  the  elec- 
trons within  the  metal,  the  internal  friction  must  also  increase, 
due  to  the  more  frequent  collisions  that  take  place  with  the  raised 
velocity,  both  between  themselves  and  with  the  violently  vibrat- 
ing ponderable  particles. 

The  mechanical  working  of  the  metal  also  causes  the  con- 
ductivity to  vary  within  certain  limits.  The  optical  properties 
are  not  thereby  altered,  accordingly  the  electron  number  remains 
constant,  while  the  frictional  resistance,  which  the  electrons 
encounter  in  their  motion,  changes. 

Thermo-electric  Force. 

Drude's  *  calculation  of  the  electron  concentration  rests  on 
a  whole  number  of  hypotheses  and  special  assumptions.  There 
are,  however,  other  magnitudes  which  stand  in  a  near  relation 
to  the  electron  concentration,  especially  the  thermo-electric 
force  of  the  metals.  These  warrant  the  possibility  of  making  a 
test  of  the  calculation. 

The  thermo-electric  force  caused  by  the  contact  of  two  metals 
may  be  considered  as  the  result  of  electron  diffusion.  It  can  be 
supposed  that,  at  the  junction  of  the  two  metals,  with  different 
electron  concentrations,  there  is  a  tendency  toward  equaliza- 
tion of  the  concentration.  However,  only  an  infinitely  small 
amount  can  actually  diffuse  into  the  metal  of  lower  electron  con- 
centration. A  loss  of  negative  electrons  leads  to  an  excess  of  pos- 
itively charged  particles,  and  thereby  electro-static  attraction 
forces  are  set  up,  which  swing  the  escaping  electrons  back  again 
to  their  original  position.  Only  a  very  small  potential  difference 
between  the  metal  remains  which  represents  the  equilibrium 
between  the  diffusion  tendency,  and  the  electro-static  attraction 
between  the  positive  particles  and  the  electrons.  The  diffusion 
tendency  depends  on  the  relation  of  the  electron  concentration 
in  the  two  metals  and  on  the  temperature.  The  potential  dif- 
ference is  accordingly  greater,  the  higher  the  temperature,  and 
the  larger  the  ratio  of  electron  concentrations  in  the  touching 

*  Ann.  Physik.,  1,  593  (1900);  14,  948  (1904). 


INTRODUCTION— PROPERTIES  OF  METALS 


33 


metals.  The  relation  is  exactly  the  same  as  with  electrolytic 
diffusion  cells. 

A  thermo-current  between  the  metals  can  only  be  brought 
about  if  two  differently  temperatured  junctions  are  present. 
The  negative  electrons  then  flow  in  the  metal  with  the  higher 
electron  concentration  from  cold  to  warm  and  in  the  other  from 
warm  to  cold.  The  metal  more  concentrated  in  electrons  is  the 
positive  pole  at  the  hot  junction. 

The  thermo-electric  force  of  a  thermo  cell  at  a  temperature 
difference  &  of  the  two  junctions  and  an  electron  concentration 
Ni  and  A/2  of  the  two  metals  is  given  according  to  Drude  by  the 
equation: 


TT  (microvolts)  = 


log,  ^4+- 
iv  2     2 


aL 


We  have  seen  above,  that  the  electron  concentration  varies 
only  slightly  with  the  temperature;  we  may,  therefore,  neglect 
the  second  portion  of  the  parenthesis  for  small  temperature 
differences  and  obtain  a  very  simple  expression. 


If  we  compare  different  metal  couples  at  the  same  tempera- 
ture difference,  in  reference  to  their  thermo-electric  force,  we  see 
that  it  should  be  entirely  determined  by  the  ratio  of  their  elec- 
tron concentrations.  The  order  of  the  metals  according  to  their 
elctron  concentrations  must,  therefore,  agree  with  the  thermo- 
electric potential  series,  and  it  must  be  possible  to  calculate  the 
thermo-electric  force  from  the  electron  concentration.  There  is 
only  a  slight  agreement  between  theory  and  fact  for  this  latter 
relation. 


Cell. 

POTENTIAL  DIFFERENCE  FOR   i° 
TEMPERATURE  DIFFERENCE. 

Calculated. 

Observed. 

Bi-Sb 

iSoXio"6 
15X10-" 

68to9iXio~6 
39Xio~6 

Fe-Co  .  .  . 

34  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  comparison  of  the  order  of  electron  numbers  and  position 
in  the  thermo-electric  potential  series  is  found  in  the  table  page  31. 
That,  in  general,  variations  occur,  is  explained  in  that  the  elec- 
tron concentrations,  calculated  from  the  optical  constants,  differ 
by  only  a  slight  amount  from  each  other.  In  all  cases,  however, 
where  great  differences  appear  in  these  numbers,  the  order  in 
both  series  is  nearly  the  same. 

In  a  general  way  the  method  laid  down  by  Drude  enables  us 
to  orient  the  metals  according  to  their  electron  concentration. 
It  cannot,  however,  be  overlooked,  that  the  single  metals,  espe- 
cially silver,  gold  and  copper,  show  very  great  variation.  It  is, 
therefore,  certain  that  the  assumptions  taken  as  the  basis  of  cal- 
culation were  not  altogether  sound. 

Gold  and  copper  being  colored  metals  undergo  a  selective 
absorption  for  certain  colors,  that  is,  wave  lengths,  therefore, 
not  only  the  free  electrons  are  concerned  in  the  optical  phenomena 
but  also  vibrations  of  the  bound  electrons.  Of  this  resonance 
phenomenon,  however,  Drude's  equation  takes  no  account.  If 
this  correction  were  made,  the  electron  concentration  would  cer- 
tainly stand  in  better  agreement  with  the  thermo-electric  data 
for  the  given  cases. 

Passivity  in  Metals. 

There  is  still  one  phenomena  which  is  characteristic  of  some 
metals  and  metallic  compounds  which  we  have  not  so  far  con- 
sidered. It  is  the  phenomenon  of  passivity. 

It  has  long  been  known  that  certain  metals  especially  iron 
were  capable  of  assuming  under  certain  conditions  a  state  of  inac- 
tivity toward  the  reagents  which  normally  attack  them  ener- 
getically. Thus  if  iron  be  immersed  in  strong  nitric  acid  it  is 
rendered  passive  and  is  no  longer  dissolved  by  dilute  acids  and 
will  not  precipitate  copper  from  solution.  Numerous  explana- 
tions have  been  offered  for  this  phenomenon,  but  the  only  one  that 
has  received  any  degree  of  acceptance  is  the  theory  of  an  oxide 
coating  which  was  first  proposed  by  Faraday.  This  theory, 
however,  is  open  to  a  large  number  of  objections.  A  theory 
of  passivity  on  the  basis  of  Drude's  electron  theory  of  metals  has 


INTRODUCTION—  PROPERTIES  OF  METALS  35 

recently  been  proposed  by  Dean  *  and  seems  to  explain  at  least 
the  outstanding  features  of  the  phenomenon.  Taking  as  a 
starting  point  the  expression  for  contact  potential  which  has  been 
derived  by  Langmuirf  where  0i  and  02  represent  the  so-called 


electron  affinities  and  Ni  and  N%  the  electron  numbers  of  the  con- 
cerned metals.  We  see  that  the  contact  potential  depends  on  N\ 
and  N%,  and  the  relation  only  holds  if  N\  arid  N2  represent  the 
electron  numbers  of  the  surface  layer.  If  it  is  possible  to  main- 
tain a  surface  of  lower  electron  concentration  than  the  main  body 
of  the  substance  we  would  have  a  case  of  a  substance 
apparently  ennobled.  Such  a  surface  condition  would  explain 
many  of  the  peculiarities  of  the  so-called  passive  metals. 

The  production  of  passivity  by  oxidizing  agents  can  be 
explained  in  this  way  since  oxidation  is  in  effect  the  removal  of 
electrons.  The  low  photo-electric  emission  of  passive  iron  as 
observed  by  Allen  {  is  in  direct  accord  with  a  surface  layer  lower 
in  electrons. 

There  remain,  however,  three  things  which  must  be  accounted 
for  to  make  an  electron  theory  of  passivity  probable  (i)  the 
maintenance  of  an  electron  deficient  surface  (2)  characteristic 
occurrence  of  passivity  in  the  metals  of  the  iron  group  (3)  the 
effect  of  magnetism  on  passivity. 

Let  us  commence  by  analyzing  mathematically  the  condi- 
tions affecting  the  surface  concentration  of  electrons.  If  a 
conductor  A,  B  with  difference  of  potential  between  A  and  B 
moves  in  a  magnetic  field  of  intensity  F,  its  velocity  normal  to 
A  ,  B  will  be  given  by 


(i) 


where  K  is  a  constant  and  /*  the  permeability  of  the  medium. 
Now,  without  interfering  with  our  analysis  we  may  replace  the 

.*Am.  J.  Sci.,  47,  123  (1919) 
f  Trans.  Am.  Electrochem.  Soc.,  p.  144  (1916). 
I  H.  S.  Allen,  Proc.  Roy.  Soc.,  88,  10  (1913). 


36  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

conductor  A,  B  with  a  stream  of  electrons  with  velocity  <r  in 
which  case  V  will  be  given  by 


Further  the  ends  of  A,  B  may  be  joined  and  all  the  electrons 
removed  but  one  and  Eq.  (2)  will  still  hold;  we  have  then  our 
ideal  case  of  one  electron  moving  in  its  orbit  with  velocity  a.  Its 
velocity  normal  to  the  plane  of  its  orbit  will  be  given  by  (2). 
The  irregular  motions  of  electrons  within  a  metal  can  be  resolved 
into  a  series  of  harmonic  motions  to  which  this  mathematical 
analysis  can  be  applied.  If  now  we  consider  the  surface  of  a  con- 
ductor we  find  that  the  electrons  on  reaching  the  second  medium 
say  air  would  have  a  velocity  of 


(3) 


where  V  is  the  velocity  in  the  conductor  and  //  the  permeability 

of  the  second  medium.     Electrons  whose  velocity  in  the  conductor 

/ 

was  greater  than  Ve—  where  Ve  is  the  velocity  of  escape,  will, 

M 

therefore,  escape.  Considering  then  a  layer  at  the  interface, 
the  number  of  electrons  coming  through  the  lower  boundary 
will  be  given  by 


where  p  is  the  electron  density  and  V  the  velocity.    The  number 
leaving  the  upper  boundary  will  be  given  by 


but  Ni=N2,  and  hence 


~,  .      V       (6) 


that  is,  the  electron  density  at  the  surface  is  less  than  that  of  the 
interior  and  is  given  by  (6).  This,  of  course,  is  only  true  for 
paramagnetic  metals  and  the  more  paramagnetic  the  substance 
the  greater  the  passivity  which  at  once  explains  the  tendency  to 
passivation  found  in  the  iron  group. 


INTRODUCTION—  PROPERTIES  OF  METALS  37 

So  far  we  have  considered  only  a  constant  magnetic  field. 
If  we  examine  Eq.  (2)  it  will  be  seen  that  V  is  inversely  pro- 
portional to  F.  Hence  it  is  apparent  that  if  a  magnetic  field  be 
applied  to  a  conductor  V  will  be  lowered  causing  the  number 
of  electrons  with  greater  velocities  than  Ve  to  be  smaller  and 
hence  the  passivity  produced  by  any  given  set  of  conditions 
will  be  less  in  a  magnetic  field  than  without,  or  the  force  neces- 
sary to  produce  a  given  degree  of  passivity  will  be  proportional 
to  F.  However  if  a  conductor  be  once  passivated  no  effect  of 
changing  the  magnetic  field  would  be  anticipated.  This  is 
practically  the  effect  found  by  Nichols  and  Franklin  *  and  by 
Byers  and  his  collaborators,  f 

The  phenomenon  of  passivity  is  not  without  meaning  in 
metallurgy  particularly  as  it  is  connected  with  contact  catalysis 
which  is  of  importance  in  the  study  of  the  blast  furnace.  The 
decomposition  of  carbon  monoxide  according  to  the  equation 


is  known  to  be  remarkably  accelerated  by  the  presence  of  iron 
or  nickel. 

Let  us  examine  the  electron  mechanism  of  this  reaction. 
Carbon  monoxide  has  probably  the  electron  structure  { 


i.e.,  the  carbon  is  bivalent.  The  first  process  in  the  mechanism 
of  the  reaction, 

2CO  -*  CO2+C 

would  be  the  increase  of  valence  of  the  carbon,  since  this  is 
the  unsaturated  atom.  This  we  may  represent 

ct:o-2  e=Jct:o. 

This  process,  therefore,  requires  the  addition  of  two  electrons. 
The  purpose  of  the  catalyst  is  to  furnish  these.  If  the  catalyst 
is  iron  we  may  write  the  reaction 

C+IO+iron  (passive)  =  +C+  10+  iron  (active). 

*  Nichols  and  Franklin,  Am.  J.  Sci.  (3),  31,  272  (1886);  (3)  34,  419  (1887). 

\  Byers  and  Darrin,  J.  Am.  Chem.  Soc.,  32,  550,  1910. 

t  Hanke  and  Koessler,  J.  Am.  Chem.  Soc.,  40,  1726  (1918). 


38  THE   PHYSICAL  CHEMISTRY   OF  THE  METALS 

The  next  step  we  may  represent 


and  finally: 

C+  +iron  active  =  C  -firon  (passive)  . 

It  should  of  course  be  pointed  out  that  an  electron  theory  of 
passivity  is  only  convenient  and  not  necessary  in  this  connec- 
tion as  the  same  result  may  be  arrived  at  by  means  of  non- 
electronic reaction  and  the  oxide  theory  of  passivity. 

Similar  explanations  may  be  worked  out  for  the  use  of  cata- 
lytic nickel  in  hydrogenations.  In  this  case  it  is  necessary  to 
start  with  active  metal  since  the  first  process  in  reduction  is  the 
addition  of  electrons. 


CHAPTER  II 
METALLIC    SOLUTIONS    AND    ALLOYS 

WE  have  learned  in  the  first  chapter  that  the  metals  show 
nothing  exceptional  in  many  of  their  properties,  that  the  law  of 
vaporization  and  melting  and  polymorphic  transition  is  the  same 
as  for  non-metallic  substances. 

The  same  conclusion  is  reached  by  the  investigation  of 
metallic  solutions  and  alloys,  since  all  the  phenomena  we  can 
observe  with  them  we  have  already  met  in  solutions  in  water 
and  other  liquids.  That  metals  can  be  dissolved  is  a  well-known 
fact.  One  needs  only  to  lay  a  thin  gold  wire  in  mercury  to  be 
very  soon  convinced  that  it  is  attacked  and  finally  dissolved. 
The  affinity  of  lead  for  the  noble  metals,  gold  and  silver,  plays 
an  important  role  in  fire  assaying,  and  everyone  who  has  worked 
in  the  laboratory,  is  familiar  with  the  destruction  which  takes 
place  when  metallic  mercury  falls  into  the  sewer  pipe. 

The  power  of  metals  to  dissolve  other  metals  is  not  confined 
to  the  liquid  state  alone.  Solid  alloys  are  also  known,  which  are 
homogeneous  solutions. 

The  solvent  action  of  the  metals  is  also  extended  to  certain 
metallic  compounds,  especially  carbides,  oxides  and  phosphides. 
Gases  can  also  be  dissolved  in  the  metals,  particularly  hydrogen. 

Colloidal  Metal  Solutions. 

In  general  the  best  solvents  for  the  metals  are  metallic  sub- 
stances.    These  real  solutions  must  not  be  confused  with  the  so- 
called  colloidal  solutions  of  metals  in  water,  and  organic  liquids 
that  have  become  known  in  great  number,  during  recent  years. 
Years  ago  the  American  chemist,  Carey  Lea,*  succeeded  in  ob- 
taining brown  solutions  of  metallic  silver  in  water,  by  reducing 
ammoniacal  silver  solutions  with  organic  reducing  agents.     In 
a  similar  way,  red  solutions  of  gold  may  be  prepared;   if,  for 
*  Phil.  Mag.,  (5)  31,  238,  320  (1891). 
39 


40  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

example,  gold  chloride  solution  is  reduced  with  formaldehyde. 
These  colloidal  solutions  can  also  be  prepared  by  the  method  of 
Bredig  *  which  consists  in  the  electrical  dispersion  of  the  metal 
by  arcing  under  water  or  other  solvent.  Such  solutions  are  not 
permanent  but  the  contents  gradually  settle  to  the  bottom,  as  a 
dark-colored  powder.  The  precipitation  is  accelerated  by  adding 
a  little  salt  or  acid  to  the  solution,  while  small  amounts  of  alkali 
seem  to  increase  the  stability.  These  colloidal  solutions  may  be 
considered  as  extremely  fine  suspensions  of  metals.  A  light  beam 
which  falls  through  such  a  liquid,  is  dispersed,  and  with  the  help 
of  the  ultra-microscope,  the  diffraction  discs  of  the  separate 
particles  can  be  recognized. 

The  gold  ruby  glass  and  its  analogues,  with  other  metals, 
belong  to  these  colloidal  solutions,  and  it  is  not  inconceivable 
that  the  small  content  of  noble  metals  which  is  found  in  certain 
volcanic  rocks  is  contained  as  a  colloidal  solution.  Some  recent 
investigations  have  suggested  that  the  yellow  and  ruby  colors  of 
sphalerite  are  to  be  attributed  to  colloidal  iron  disulfide.f 

The  experiments  of  R.  Lorenz  {  indicate  that  the  tendency 
of  metals  in  contact  with  liquid  melts,  to  disperse  into  such 
extremely  fine  suspensions  or  fogs  is  very  great.  This  phe- 
nomenon is  shown  nicely  by  the  following  experiment  taken  from 
Lorenz. 

If  a  salt,  for  example,  lead  chloride,  be  melted  in  a  reagent 
tube  and  a  piece  of  metal,  for  example,  lead,  be  dropped  into  it, 
the  metal  at  first  melts  to  a  ball,  but  after  a  few  seconds  it  dis- 
perses suddenly  to  a  metal  fog,  spreading  rapidly  throughout 
the  whole  melt  and  coloring  it  dark.  The  melt  appears  trans- 
parent in  transmitted  light  but  opalescent  or  opaque  in  incident 
light. 

This  phenomenon  sometimes  causes  difficulty  in  the  electrol- 
ysis of  molten  salts,  fogs  forming  at  the  cathode,  which  wan- 
der to  the  anode,  and  there  enter  again  into  combination  (e.g., 
with  the  liberated  chlorine).  The  appearance  of  such  fogs  is 

*  Z.  Elektrochem.,  4,  514,  547  (1898). 
t  Bull.,  Mo.  Sch.  of  Mines,  3  (1915). 
J  Z.  Elektrochem.,  13,  582  (1907). 


METALLIC  SOLUTIONS  AND  ALLOYS 


41 


especially  common  with  cadmium  and  also  with  the  alkali 
metals.  In  the  latter  case,  the  electrolyte  on  cooling,  shows  an 
intense  blue  color,  which  appears  to  be  due  to  the  mixing  of  the 
sodium  fog.  The  blue  color  of  a  solution  of  sodium  in  liquid 
ammonia  is  probably  also  due  to  a  fine  dispersion.  Siedentopf  * 
has  also  shown  that  the  blue  color  of  rock  salt  is  best  explained 
as  due  to  the  dispersion  of  sodium  metal  through  the  mass.  A 
real  solution  of  a  metal  in  a  non-metallic  solvent  is  entirely 
abnormal. 

Dilute  Metallic  Solutions. 

A  dilute  melt,  formed  by  dissolving  a  small  amount  of  one 
metal  in  another,  shows  all  the  characteristics  of  a  dilute  solu- 
tion. Particularly  characteristic  of  real  solutions  is  their 
"  dilution  tendency  "  and  with  it  are  connected  most  of  the 
properties  of  a  solution.  If  solutions  of  two  different  concen- 
trations are  stratified  one  over  the  other,  an  adjustment  takes 
place;  the  dissolved  substance  diffusing  from  the  place  of  higher 
concentration  to  that  of  lower.  The  speed  with  which  this  dif- 
fusion takes  place,  is  of  the  same  order  of  magnitude  as  with 
aqueous  solutions.  The  diffusion  constants  k  for  some  of  the 
metals  are  given  in  the  following  table.  The  values  are  taken 
from  the  "  Physikalische  Chemische  Tabellen "  of  Landolt, 
Bornstein  and  Meyerhoffer. 


Substance. 

Temp. 
Deg. 

k  cm. 
Day. 

Observer. 

Lead  in  mercury 

I    37 

G  IVIeyer 

Lead  in  tin  

5OO 

3    18 

Roberts-  Austin 

Cadmium  in  mercury  

i  <;6 

G  Meyer 

Gold  in  mercury 

II 

O    72 

Roberts-  Austin 

Gold  in  lead  

400 

3    O3 

Roberts-  Austin 

Gold  in  lead  

"?OO 

7      IQ 

Roberts-  Austin 

Gold  in  bismuth 

COO 

A      C2 

Roberts-  Austin 

Gold  in  tin  

c,oo 

4.  6c. 

Roberts-  Austin 

Platinum  in  lead  

4QO 

I    60 

Roberts-  Austin 

Rhodium  in  lead    .  . 

coo 

3   O4 

Roberts-  Austin 

Silver  in  tin  

«;OO 

4-  *4 

Roberts-  Austin 

Zinc  in  mercury  

2    OO 

G.  Meyer 

*  Physik.  Z.,  6,  320,  1915. 


42  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  diffusion  coefficient  of  normal  sulfuric  acid  is  given  by 
Arrhenius  as  1.12  in  the  units  of  the  table  and  that  of  double 
normal  as  1.16  both  at  12°. 

The  dilution  tendency  can  perform  work  and  overcome 
resistance.  It  offers  an  explanation  of  the  osmotic  phenomena. 
If  a  solution  be  conceived  of,  as  in  a  vessel  with  a  semi-permeable 
bottom  and  immersed  in  the  pure  solvent;  the  solution  could 
become  diluted,  since  the  solvent  could  be  taken  in  through  the 
semi-permeable  membrane.  If  the  solution  be  placed  under 
pressure,  by  means  of  a  movable  weighted  piston,  the  solvent 
would  be  forced  out  again,  and  a  concentration  effected.  The 
dilution  tendency  will  accordingly  oppose  work,  and  there  must 
be  a  counter  pressure  which  the  dilution  tendency  will  hold  in 
equilibrium,  so  that  the  solvent  neither  goes  in  nor  comes  out  of 
the  cell.  This  equilibrium  pressure  is  designated  as  the  osmotic 
pressure  of  the  respective  solutions.  Its  magnitude  depends 
on  the  temperature  and  the  number  of  dissolved  molecules  which 
the  space  contains,  and  in  fact  the  osmotic  pressure  is  equal  to 
the  pressure  which  a  substance  would  exert  as  a  gas  at  the  same 
temperature  and  molecular  concentration. 


Vapor  Pressure  of  Metallic  Solutions. 

If  a  solution  occurs  in  contact  with  the  vapor  of  its  solvent 
the  upper  surface  of  the  liquid  may  be  considered  as  a  semi- 
permeable  membrane,  through  which  the  molecules  of  the  sol- 
vent, but  not  of  the  solute,  may  pass  to  the  vapor  space.  Two 
opposing  forces  are  now  to  be  considered:  the  tendency  of  the 
solvent  to  evaporate  and  the  tendency  of  the  solution  to  dilute 
itself  by  absorbing  solvent  molecules  from  the  vapor  space. 
An  equilibrium  is  established  between  these  two  forces  which 
act  on  the  vapor,  and  the  pressure  corresponding  to  this  equi- 
librium is  the  vapor  pressure  of  the  solution.  The  measurable 
effect  of  the  dilution  tendency  is  the  lowering  of  the  vapor 
pressure  of  a  solution,  as  compared  to  the  pure  solvent.  The 
vapor  tension  of  a  dilute  solution  is  always  lower  than  that  of  the 
pure  solvent.  The  lowering  of  the  vapor  pressure  depends  as 


METALLIC   SOLUTIONS  AND  ALLOYS 


43 


does  the  magnitude  of  the  osmotic  pressure,  on  the  molecular 
concentration  and  the  temperature;  it  is  likewise  a  function 
of  the  dilution  tendency.  The  vapor  pressure  of  the  solution 
being  lower  than  that  of  the  pure  solvent,  the  boiling  point,  that 
is,  the  temperature  at  which  the  vapor  pressure  is  equal  to 
atmospheric,  is  accordingly  higher.  These  relations  can  be  at 
once  recognized,  from  the  graphical  representation,  as  shown  in 
Fig.  ii.  The  only  metallic  solutions  feasible  to  study  are  those 
with  mercury,  the  lowest  boiling  of  the  metals. 


760 
m  m. 


Temperature 


F/    F 


M'M 


FIG.  ii. 


Investigations  on  the  vapor  pressure  lowering  of  dilute 
amalgams,  have  been  carried  out  by  Sir  William  Ramsay  *  for 
the  purpose  of  determining  the  molecular  weights  of  the  dis- 
solved metals.  As  we  have  seen,  the  vapor  pressure  lowering  is 
proportional  to  the  number  of  dissolved  molecules.  Accord- 
ingly if  we  know  the  depression  for  a  dissolved  substance  of 
known  molecular  weight,  we  can  calculate  the  molecular  weight 
of  other  substances  from  the  vapor  pressure  depression  observed 
for  them. 


*  J.  Chem.  Soc.,  55,  521  (i 


44 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


The  results  of  Ramsay's  observations  are  given  in  the  fol- 
lowing table: 


Temp. 

Dissolved 
Metal. 

No.  Atoms 
per  100  At. 
Hg. 

V.  P.  Lower- 
ing  in  Mm. 

Calculated 
Molecular 
Weight. 

Atomic 
Weight. 

Li 

1.70 

12.9 

7-1 

7.02 

• 

Na 

/o.86 

6.7 

81.61 

2 

li.87 

17.9 

i8.3/ 

K 

(1-55 
\5-26 

15-6 
49.6 

29.1  1 
30.  2  / 

39-15 

Ca 

o.  19 

2.9 

19.1 

40.1 

Ba 

0.90 

12.3 

75-7 

137.4 

b 

Mg 

f  0.70 
\4-82 

5-4 
40.6 

24.  o  j 
21.  5  J 

24.36 

V 

r  1.07 

7-4 

70.1  \ 

.  s 

Zn 

J 

15-5 

62.1  I 

65.4 

p 

[3.16 

23.6 

65-  4J 

•  l  .       J 

fo.75 

6-3 

IOO.  2  \ 

•3 

Cd 

\ 

16-3 

99-7  [ 

112.4 

"o 

U'si 

27.4 

103.  8J 

£ 

0.8  1 

7.6 

163.6 

.s 

Tl 

1.66 
2.92 

13-3 
24.6 

K86.8 

180.1 

204.1 

pq 

3-33 

25-3 

197.6] 

Sn 

1.94 

14.7 

117.4 

119.0 

Pb 

2.OO 

15-4 

199.9 

206.9 

Bi 

3-77 

26.8 

214-5 

208.0 

Mn 

1.14 

8-5      ' 

55-5 

55-0 

Ag 

3.22 

22.8 

112.4 

107.93 

Au 

f  1-59 
I  2.  80 

n-3 
19.2 

207.4! 
208.1  ( 

197.2 

There  are  two  things  to  be  noted  from  this  table,  first,  the 
proportionality  between  concentration  and  depression  and 
second,  the  confirmation  of  the  fact  that  most  of  the  metal 
molecules  are  monatomic,  which  can  also  be  observed  in  vapor 
density  determinations. 

Freezing  Point  of  Alloys. 

The  vapor  pressure  is  also  related  to  the  freezing  point  of 
solutions.  The  freezing  point  of  a  solution  is  always  lower 
than  that  of  the  pure  solvent;  it  is  a  recognized  fact,  for  example, 
that  salt  water  is  very  much  more  difficult  to  freeze  than  pure 


METALLIC  SOLUTIONS  AND  ALLOYS 


45 


water.  In  the  first  chapter  we  showed  that  the  freezing  point 
was  the  intersection  of  the  vapor  pressure  curve  for  the  solid 
substance  and  that  for  the  liquid.  Through  the  addition  of 
the  dissolved  substance  the  vapor  pressure  curve  of  the  liquid  is 
lowered.  The  vapor  pressure  of  the  solid  substance  is  in  general 
uninfluenced  by  the  dissolved  substance  since  the  solvent  pre- 
cipitates out  in  the  pure  state.  The  freezing  point  of  a  solution 
is  accordingly  principally  dependent  on  the  vapor  pressure 
curve  of  the  liquid  and  is  hence  depressed.  The  magnitude  of 
the  depression  depends  with  the  same  solvent,  on  the  molecular 
concentration,  precisely  as  with  the  lowering  of  the  vapor  pres- 
sure and  the  raising  of  the  boiling  point. 

There  is  a  very  great  amount  of  experimental  data  on  the 
freezing  point  lowering  of  metallic  solutions.  The  work  of  Hey- 
cock  and  Neville  *  and  of  Tammann  f  has  been  of  particular 
value  in  this  field.  The  magnitude  of  the  depression  which  a 
gram  molecule  causes,  when  dissolved  in  100  grams  of  solvent,  is 
called  the  molecular  freezing  point  lowering,  and  can  be  calcu- 
lated from  the  thermal  constants  of  the  .solvent,  its  freezing 
point  (in  absolute  degrees),  and  its  heat  of  fusion.  The  depres- 
sion is  connected  with  these  magnitudes  through  the  following 
equation  which  is  due  to  Van'T  Hofif  , 

T2 

A  =  0.02  —  . 


T  is  the  absolute  melting  point,  q  the  heat  of  fusion. 

The  following  table  gives  these  magnitudes  for  a  series  of 
metals: 


Metal. 

A. 

Metal. 

A. 

Antimony  
Lead 

I24O 

122^ 

Silver  
Bismuth 

1443 
460 

Cadmium  

516 

Tin  

•JCQ 

Copper.  .  . 

870 

Zinc  

34.1 

Mercury  

3QO 

*J.  Chem.  Soc.,  61,  904  (1892);    55,  666  (1889);    57,  376  (1890);    71,  383 
(1897);  Trans.  Roy.  Soc.,  189,  A25  (1897). 
t  Z.  Physik.  Chem.,  3,  441  (1898). 


46 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


Through  comparison  of  these  numbers  with  the  observed 
depression,  the  molecular  weight  of  the  dissolved  metal  can  be 
determined.  Experiments  of  this  nature  have  also  confirmed 
the  fact,  that  in  most  cases  the  metals  are  monatomic.  The 
data  is  given  in  the  following  table: 


Solvent. 

Solute. 

N. 

Solvent. 

Solute. 

N. 

Arsenic 

I 

Copper 

2 

Gold     

I 

Sodium  

Lead  

Copper 

I 

Palladium  .... 

Silver 

I 

Bismuth 

<  Platinum 

Bismuth  

2 

Mercurv  

Arsenic  .  .  .  '.  .  . 

I 

Silver  

Lead 

I 

I  Tin 

Gold 

Lead 

Cadmium  

Sodium  
Palladium  .... 
Platinum 

I 
2 

I 

Tin 

Gold  
Copper  
<  Magnesium 

IMercury 

2 

Sodium 

Thallium   .  .    . 

I 

Nickel  

Bismuth 

I 

Palladium  . 

Tin 

I 

Silver 

Mercury  

f  Thallium  
I  Zinc  .  . 

I 
I 

Thallium  
Antimony  .  .    . 

(?) 

(  Aluminum 

2 

Lead 

Zinc  

< 

Silver         .  . 

J  Tin  . 

2 

1  Magnesium.  .  . 

1  Zinc  
c  Arsenic 

2 

•2 

Thallium  
Bismuth    .... 

Bismuth 

•1  Cadmium  .... 

I 

Tin  

[  Gold  

I 

Metals  only  Partially  Miscible  in  Liquid  State. 

The  concentrated  solutions  of  metals  also  show  all  the  phe- 
nomena that  occur  with  non-metallic  liquids.  There  are  liquid 
metals  that  mix  with  one  another  in  all  proportions,  for  exam- 
ple, lead  and  tin,  and  there  are  those  that  are  only  partially 
miscible,  for  example,  lead  and  zinc.  If,  for  example,  lead  and 
zinc,  or  zinc  and  bismuth  are  melted  together,  the  melt  separates 
into  two  liquid  layers,  exactly  as  in  the  case  of  water  and  ether. 
The  composition  of  the  two  layers  is  different  and  becomes  more 
nearly  equal  with  rising  temperature,  finally  it  becomes  identical 


METALLIC  SOLUTIONS  AND  ALLOYS 


47 


for  both  layers,  and  the  layers  disappear.  The  temperature  of 
this  complete  miscibility  is  called  the  critical  temperature. 
With  a  mixture  of  bismuth  and  zinc  this  critical  temperature  is 
about  820°,  for  lead  and  zinc  about  935°.  Above  these  temper- 
atures the  miscibility  is  complete,  below  it  is  only  partial.  In 
the  case  of  layer  formation  every  temperature  corresponds  to  a 
definite  composition  of  the  two  layers.  This  inter-dependence 
is  represented  graphically  by  a  curve  with  two  limbs  which  inter- 
sect at  the  critical  temperature.  Outside  of  this  curve  is  the 
field  for  homogenous  liquid  mixtures;  inside  a  homogenous  mix- 
ture separates  into  two  liquid  layers.  The  composition  of  the 
layers  at  different  temperatures  is  shown  in  the  following  table 
(according  to  Spring  and  Romanoff).* 


BlSMUTH-ZlNC. 

LEAD-ZINC. 

Temp. 

Under  Layer. 

Over  Layer. 

Under  Layer. 

Over  Layer. 

Per  Cent 

Per  Cent 

Per  Cent 

Per  Cent 

Per  Cent 

Per  Cent 

Per  Cent 

Per  Cent 

Bi. 

Zn. 

Bi. 

Zn. 

Pb. 

Zn. 

Pb. 

Zn. 

266 

86.0 

14.0 

7  -2  A 

98.8 

I    2 

4.10 

3  o 

07  o 

I    r 

98  «: 

450 

92.0 

8.0 

475 

84.0 

16.0 

S-o 

95-o 

91.0 

9.0 

2.0 

98.0 

CIA 

80  o 

II    OO 

3O 

O7   O 

584 

80.0' 

20.  o 

IO.O 

90.0 

8^.0 

14.0 

5-0 

95-o 

650 

77.0 

23.0 

15-0 

85.0 

83.0 

17.0 

7.0 

93-o 

74.O 

70  O 

21    O 

IO   O 

QO   O 

750 

70.0 

30.0 

27.0 

73-0 

800 

7^   O 

2S    O 

14   O 

86  o 

900 

59-0 

4I.O 

25-5 

74-5 

The  Parkes  Process. 

An  interesting  use  is  made  of  the  partial  miscibility  of  lead 
and  zinc,  in  the  Parkes  process  for  the  desilverization  of  base 
bullion.  A  small  amount  of  zinc  is  added  to  the  molten  lead, 
held  at  about  500°,  and  the  mixture  stirred.  A  lighter  layer 
collects  on  the  upper  surface  which  contains  97  per  cent  Zn  and 
*  Z.  anorg.  Chem.,  13,  29  (1897). 


48 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


3  per  cent  Pb.  In  it  is  found  most  of  the  silver  originally  in  the 
lead.  The  silver  has  been  extracted  from  the  lead  by  the  zinc. 
This  process  reminds  one  of  the  shaking  out  with  ether,  com- 
monly used  in  organic  chemistry,  to  extract  dissolved  sub- 
stances from  water  by  their  solubility  in  ether.  Both  processes 
depend  on  the  fact,  that  the  third  substance  distributes  itself 
between  the  layers  according  to  a  definite  ratio. 

The  ratio  of  the  concentrations  of  the  dissolved  substance 
is  designated  as  the  partition  coefficient.     This  magnitude  is 


900 
800° 
700° 
600° 
500° 
400° 


0 
100 


Complete  Miscibility 


PartiaLMiscibility 


10 
90 


20 


40 

GO 


.50 
50 

FIG.  12. 


GO 
40 


70 


900 

800° 

700° 

600° 

500° 

400* 


100 
0 


large  for  silver  distributed  between  zinc  and  lead.  The  rela- 
tion is  well  illustrated  by  an  analagous  experiment  with  iodine 
water  which  is  shaken  with  a  small  amount  of  ether.  On  set- 
tling the  ether  is  found  to  contain  the  principal  amount  of  the 
iodine,  which  can  be  recognized  by  its  brown  color.  The  iodine 
water  represents  the  argentiferous  lead  and  ether  the  added 
zinc.  It  can  be  seen  at  once  that  it  is  not  possible  to  remove 
the  dissolved  substance  entirely  by  means  of  the  partition  process 
without  repeating  the  process  a  number  of  times. 


METALLIC   SOLUTIONS  AND  ALLOYS 


49 


Solidification  Curve  of  Binary  Alloys,  the  System  Cadmium-Zinc. 

On  cooling  a  homogenous  melt,  there  first  begins  a  separation 
of  solid  crystals,  and  finally  the  entire  mass  solidifies.  We  will 
now  study  the  solidification  phenomena  of  an  alloy  made  up  of 
two  metals. 

We  will  first  consider  the  metals  zinc  and  cadmium.  The 
crystallization  point  of  pure  zinc  is  at  419.4°  .  If  small  amounts 
of  cadmium  be  added  to  this  metal,  the  crystallization  point  will 
be  depressed,  according  to  the  above-stated  principle,  and  the 
further  the  greater  the  concentration  of  the  second  metal.  If 


Liquid  Melt 


lelt 


Eutectic 

+Za  Crystals 


Eutectic 


Cd  Crystals 


450 
400 
350 
300 
250 
200 


450 
400 
350 
300 
250 
200 


0    10    20    30    40    50    60    70    80    90   1( 
100   90   80    70   60    50   40    30   20    10   '  0  %  Zn 

FIG.  13. 

now  we  represent  graphically  the  dependence  of  the  solidifica- 
tion point  on  the  composition  of  the  melt,  we  obtain  a  curve 
which  begins  at  the  melting  point  of  the  pure  substance  and 
gradually  sinks.  There  may  be  obtained,  however,  a  similar 
curve  by  starting  with  pure  cadmium  and  adding  zinc  to  the 
melt.  The  zinc  and  cadmium  crystallize  out  of  the  melt  in  the 
pure  state,  no  compounds  forming,  nor  mixed  crystals  separating. 
This  system  represents  the  simplest  possible  case. 

The  two  curves  represent  respectively,  the  equilibrium  be- 
tween liquid  zinc-cadmium  melt  and  pure  zinc  crystals,  and  the 


50 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


equilibrium  between  the  melt  and  pure  cadmium  crystals.  The 
two  curves  intersect  at  a  definite  temperature  and  a  definite 
composition  of  the  melt.  At  this  temperature  zinc  and  cadmium 
crystallize  simultaneously  and  the  melt  solidifies  completely. 
This  point  of  complete  crystallization  is  designated  as  the  eu- 
tectic  point.  For  the  zinc  cadmium  alloys  it  is  at  264°  and  the 
eutectic  alloy  contains  26  atomic  per  cent  of  zinc  (i.e.  17.35  per 
cent  by  weight). 

The  eutectic  temperature  is  lower  than  the  melting  point  of 
either  of  the  components.  It  is  marked  by  a  retardation  of  the 
cooling  in  an  entirely  similar  way  to  the  point  of  arrest  of  the 


10      15       20       25       30       35       40 


FIG.  14. 


pure  metals,  the  temperature  remaining  steady  till  the  melt  is 
completely  solidified  to  a  solid  alloy. 

The  schematic  representation  of  a  series  of  cooling  curves 
for  mixtures  of  the  same  components  (salt  and  water)  but  in 
different  proportions  is  shown  in  Fig.  14. 

Two  points  of  arrest  can  be  recognized  in  these  curves,  an 
upper,  which  indicates  the  beginning  of  the  precipitation  of  a 
component,  its  position  depending  on  the  composition,  and  a 
lower,  namely,  the  eutectic  point,  which  has  the  same  position 
for  all  compositions  of  the  mixture. 

A  large  number  of  metal  pairs  are  similar  to  cadmium  and 
zinc.  The  position  of  the  eutectic  point,  the  melting  point  of 
the  alloy  components  and  the  composition  of  the  eutectic,  are 


METALLIC   SOLUTIONS  AND  ALLOYS 


51 


however,  quite  different,  as  shown  by  the  following  table,  taken 
from  Roozeboom.* 


COMPONENTS. 

EUTECTIC. 

A. 

M.  P. 

B. 

M.  P. 

Temp. 

Cone.  B 
(Atom- 
Per  Cent). 

I 

Ag 

960 

Cu 

1081 

778 

40 

2 

Cd 

320 

Zn 

418.5 

264 

26.5 

3 

Sn 

232 

Pb 

322 

183 

23 

4 

Sn 

232 

Ag 

960 

221 

3-8 

5 

Zn 

418 

Al 

654 

38l 

ii 

6 

Pb 

328 

Ag 

960 

303 

4-4 

7 

Tl 

301 

Cd 

320 

203-5 

27.2 

8 

Sn 

232 

Zn 

418.5 

198 

16 

9 

Sn 

232 

Cd 

320 

I78 

31-2 

10 

Cd 

320 

Pb 

322 

249 

67-3 

ii 

Bi 

268 

Cd 

320 

149 

55-7 

12 

Sn 

232 

Tl 

301 

I7O.2 

3i 

13 

Sn 

232 

Bi 

268 

135 

42 

14 

Pb 

322 

Sb 

632 

228 

20.5 

15 

Bi 

268 

Pb 

322 

"5 

43-8 

Still  more  important  than 
the  various  relations  of  the 
different  metals  given  in  the 
table,  are  those  brought  out 
by  the  equilibrium  curves. 
Two  such  pairs  are  repre- 
sented in  Fig.  15,  the  pair 
Cu,  Ag  and  the  pair  Ag,  Pb. 
In  the  first,  the  eutectic  mix- 
ture consists  of  60  atomic  per 
cent  silver  and  40  atomic  per 
cent  copper  and  the  eutectic 
temperature  is  200°  below 
the  melting  point  of  the  lower 
melting  constituent;  with  the 
second  pair,  the  difference 
between  the  eutectic  point 


1100 

•Cu 
1000- 

900' 
800- 
700- 
600- 
500- 

.400' 

Pb 

300 

200* 


Ag 


20        40        60        80       100  Ag 


FIG.  15. 
*  Heterogene  Gleichgewitche,  Heft  n,  p.  196. 


52  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

and  the  melting  point  is  only  25°  and  the  silver  content  of  the 
eu  tec  tic  melt  is  very  small  being  only  4.4  atomic  per  cent  or  2.6 
per  cent  by  weight. 

The  Pattison  Process. 

These  relations  are  of  importance  since  we  have  seen  that 
commercial  lead  is  argentiferous.  The  content,  however,  is  gener- 
ally small,  and  before  separating  the  silver  and  lead  by  cupelling, 
it  is  necessary  to  enrich  the  lead  in  the  noble  metal.  This  can 
be  done  by  the  Pattison  process,  in  which  the  temperature  of 
the  molten  lead  is  lowered  so  far,  that  a  precipitation  of  the  lead 
takes  place,  the  precipitated  crystals  consisting  of  pure  lead  are 
skimmed  off,  the  silver  remaining  in  the  liquid,  and  enriching  it. 
It  is  not  possible  by  this  process,  to  obtain  a  silver  concentration 
of  more  than  2.6  per  cent.  The  concentration  of  the  eutectic 
mixture  forms  the  upper  limit  and  the  alloy  must  not  be  allowed 
to  cool  past  the  eutectic  point,  or  the  whole  mass  will  solidify. 

Segregation  in  Alloys. 

If  we  consider  the  zinc-cadmium  diagram  (Fig.  13),  we  see 
that  the  field  is  divided  by  the  two  curves  and  the  ordinate  that 
passes  through  the  intersection.  Between  the  curves  and  the 
abscissa  at  264°  is  a  field  inside  of  which  homogenous  mixture 
of  the  two  metals  cannot  exist.  They  break  up  into  solid  crystals 
and  a  melt,  the  composition  of  which  is  expressed  by  a  point  on 
the  curve.  The  crystals  which  separate,  are  of  one  component 
on  one  side  of  the  ordinate,  through  the  eutectic  point,  and  of 
the  other  component  on  the  other  side. 

The  crystals  precipitated  from  a  liquid  alloy  frequently 
possess  a  different  specific  gravity  from  the  melt,  and  hence  rise 
to  the  top  or  sink  to  the  bottom.  If  now  the  alloy  be  cooled 
gradually,  below  the  eutectic  point,  and  different  layers  of  the 
solid  alloy  examined,  it  is  found  that  they  vary  markedly  in 
composition.  Above  or  below  there  is  probably  one  of  the  pure 
components,  and  in  the  middle  the  composition  approaches  that 
of  the  eutectic  alloy.  This  kind  of  stratification  in  an  alloy  is 
designated  as  segregation.  Such  a  segregation  is  impossible  if 


METALLIC  SOLUTIONS  AND  ALLOYS  53 

the  alloy  has  the  composition  of  the  eutectic  mixture  as  this 
crystallizes  uniformly.  The  crystals  of  the  two  components 
separate  simultaneously  and  are  fine  and  uniformly  distributed. 

While  the  eutectic  crystallizes  finely,  the  crystals  of  the  excess 
component  have  time  while  swimming  in  the  melt  during  cooling, 
to  develop  fully  in  all  directions.  The  result  is,  that,  on  com- 
plete solidification,  the  alloy  consists  of  large  crystals,  embedded 
in  finely  crystalline  eutectic.  The  number  of  scattered  crystals 
become  less  as  the  total  composition  of  the  alloy  approaches  that 
of  the  eutectic. 

The  diagram  (Fig.  13)  shows  the  field  in  which  the  solid  alloy 
consists  of  eutectic  with  zinc  crystals  on  the  one  hand  and 
eutectic  with  cadmium  on  the  other. 

Investigation  of  Alloy  Structure. 

Various  processes  are  now  employed  for  the  investigation  of 
alloy  structure,  which  has  already  become  important  in  deter- 
mining the  physical  properties  of  alloys. 

We  may  consider  first  the  method  of  Heycock  and  Neville  * 
in  which  alloys  are  penetrated  by  means  of  X-rays.  It  is, 
however,  dependent  on  a  condition  which  is  fulfilled  by  only  a 
few  alloys,  namely,  the  components  must  be  very  differently 
transparent  to  Roentgen  rays.  The  method  has  proved  itself 
useful  with  alloys  of  metals  of  high  atomic  weight  with  those  of 
low  atomic  weight,  since  there  exists  a  parallelism  between  the 
absorption  of  X-rays  and  the  atomic  weight.  The  two  English 
metallurgists  investigated  in  this  way  gold-aluminum  and  gold- 
sodium  alloys.  The  latter  pair  has  a  eutectic  temperature  of 
82°  and  the  eutectic  mixture  has  a  gold  content  of  3.5  per  cent. 
The  radiographs  of  these  alloys  is  shown  in  Fig.  16,  a,  b,  c. 

Radiograph  Fig.  i6a  shows  the  structure  of  a  eutectic  mix- 
ture, Fig.  1 6b  represents  an  alloy  with  excess  sodium,  the  fern- 
like  forms  of  X-ray  transparent  crystals  which  appear  light  in 
the  positive  are  easily  recognized,  as  are  the  opaque,  hair-like 
gold  crystals  in  Fig.  i6c.  However,  in  both  the  latter  pictures, 
the  presence  of  the  embedding  eutectic  is  perceived. 

*  J.  Chem.  Soc.,  73,  714(1898). 


54 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


The  microscopic  investigation  of  alloys  in  reflected  light,  is 
useful  to  a  much  greater  extent.  The  process  has  been  developed 
especially  by  Sorby,  Martens,  LeChatelier,  Heyn,  Tammann  and 


^^^^m^^ 


a  b  c 

FIG.  1 6. — Radiographs  of  Au-Na  Alloys.     (Heycock  and  Neville.) 

others.  The  alloys  are  cut,  polished  and  etched,  so  as  to  bring 
a  single  component  into  relief.  The  following  pictures  show 
a  number  of  zinc-cadmium  alloys  so  treated. 


FIG.  17.— Zn-Cd  Alloy,  Eutectic. 


Fig.   17  shows  the  eutectic  alloy    with  slight  enlargement. 
The  light  particles  are  cadmium,  the  dark  zinc.     Fig.  18,  shows  a 


METALLIC  SOLUTIONS  AND  ALLOYS  55 

cadmium  rich  alloy,  the  bright  cadmium  stars  surrounded  by 
eutectic.  Fig  19  shows  a  zinc  rich  alloy  of  large  feathered  zinc 
needles,  in  a  light  ground  mass  of  eutectic. 

The  structure  of  the  eutectic  alloy  is  only  resolvable  with 
strong  magnification.  Fig.  20  shows  in  a  cadmium  zinc  alloy, 
the  characteristic  appearance  of  the  eutectic  structure.  It 
shows  clearly,  how  dark  zinc  layers  and  light  cadmium  layers, 
alternate  with  each  other.  The  eutectic  structure  is  a  lamellar 
one. 


FIG.  1 8.— Zn-Cd  Alloy,  Eutectic  with  Embedded  Cadmium  Crystals. 

Solid  Solution. 

In  the  alloys  previously  considered,  the  components  precipi- 
tate in  the  pure  state.  There  are,  however,  a  large  number  of 
metals  between  which  miscibility  in  the  solid  state  exists.  With 
these,  solid  solutions  are  precipitated  from  the  cooling  melt. 
There  are,  with  solids  as  with  liquids,  cases  of  total  and  partial 
miscibility.  \ 

Many  metals  form  solid  solutions  in  all  proportions,  impor- 
tant examples  being  the  coinage  alloys,  gold-silver  and  copper- 


56  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


FIG.  19  — Zn-Cd  Alloy,  Eutectic  with  Embedded  Zinc  Crystal& 


FIG.  20.— Zn-Cd  Alloy,  Eutectic  Greatiy  Enlarged. 


METALLIC  SOLUTIONS  AND  ALLOYS  57 

nickel.  Tammann  *  gives  a  compilation  of  the  single  cases  in 
which  complete  miscibility  exists,  they  are,  Cu-Mn,  Cu-Ni, 
Ag-Au,  Mg-Cd,  Bi-Sb,  Mn-Fe,  Mn-Ni,  Mn-Co,  Fe-Co, 
Fe-Ni,  Ni-Co,  Pt-Cu,  Pt-Au,  B-Cu,  B-Ag,  Pd-Au,  In-Pb. 

The  Heraus  Process  of  Plating  Platinum  with  Gold. 

Solid  solutions  are  to  be  considered  as  true  solutions,  despite 
their  solid  state  of  aggregation.  The  phenomena  characteristic 
of  solutions  can  be  observed  in  them,  especially  the  diffusion  of  a 
dissolved  substance  from  a  place  of  higher  concentration  to 
one  of  lower.  This  is  shown  in  the  Heraus  process  of  plating 
platinum  with  gold,  in  which  a  platinum  block  is  heated  to  the 
melting  point  of  gold  and  liquid  gold  is  poured  on  it.  On  cooling 
the  two  are  so  solidly  united  that  the  combination  can  be  rolled 
to  thin  leaves.  Further,  strong  temperature  changes  do  not 
affect  the  solidity  of  the  combination,  in  spite  of  the  very  differ- 
ent expansion  of  the  two  metals.  This  process,  so  valuable 
practically,  is  only  possible,  since  gold  can  diffuse  into  the  solid 
platinum,  and,  indeed,  with  considerable  speed,  so  that  the  gold 
does  not  come  into  direct  contact  with  the  platinum,  but  there  is  a 
continuous  transition  of  gold  through  solid  gold-platinum 
solutions,  the  concentration  of  which  diminishes,  till  finally  the 
pure  platinum  is  reached. 

Crystallization  Diagrams  for  Metals  Forming  Solid  Solutions. 

The  crystallization  diagram  of  a  metal  pair  of  this  kind  dif- 
fers very  essentially  from  those  already  treated.  The  lack  of  a 
eutectic  point  is  characteristic.  The  form  of  the  curves  is  shown 
in  Fig.  21,  for  the  pair  Pt-Au,  in  Fig.  22  for  the  system  Au-Ag, 
and  in  Fig.  23  for  the  solid  solutions  of  Bi-Sb. 

Figs.  21  and  22  show  the  composition  of  the  melt,  at  the  start 
of  the  solidification  but  give  no  information  as  to  the  composi- 
tion of  the  solid  solutions  which  separate  from  the  melt.  The 
compositions  of  the  liquid  and  the  solid  are,  with  special  excep- 
tions which  we  will  take  up  later,  entirely  different. 

*  Z.  anorg.  Chem.,  53,  447  (1907). 


58 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


If  we  plot  the  melting  temperatures  of  the  solid  alloys  against 
their  composition,  two  curves  are  obtained  which,  however, 
coincide  with  the  crystallization  curves  only  at  the  melting 
points  of  the  components.  Fig.  23  shows  the  position  of  the  two 


J.8UU 

1775 
1700 

1600 
1500 
1400 
1300 
1200 
1100 
innn 

X 

Pt 

X 

*>v 

s 

x 

X 

s 

x 

X 

x 

x 

Au 

1023 


FIG.  21. 

curves  for  Bi-Sb  alloys.  Curve  Bi-A-Sb  is  the  line  for  the 
beginning  of  melting,  curve  Bi-C-B-Sb  is  the  line  for  the  begin- 
ning of  solidification.  The  two  curves  enclose  a  field  outside  of 
which  solutions  in  either  the  liquid  and  the  solid  state  can  exist. 
In  the  gap  only  heterogenous  mixtures  of  solid  and  liquid  solu- 


Au 

1060 

i 



—  ==; 

1  —  * 

1040 

v^ 

X 

1020 

\ 

1000 

V 

\ 

980 

! 

\K 

0  10  20  30  40  50  60  70  80  90  100 
FlG.  22. 


960 


tions  are  possible.  As  the  temperature  is  lowered  the  first 
crystals  to  precipitate  are  rich  in  antimony  but  as  the  tempera- 
ture is  lowered  the  bismuth  content  grows  steadily.  The  com- 
pletely solidified  alloy  is  accordingly  not  homogenous,  but  con- 
sists of  a  stratification  of  crystals  of  different  composition.  In 


METALLIC  SOLUTIONS  AND  ALLOYS 


59 


some  cases  it  is  possible  to  make  the  alloys  homogenous,  by  sup- 
plementary heating,  at  a  temperature  near  the  melting  point, 
when  an  adjustment  of  the  concentration  takes  place  by  dif- 
fusion. This  diffusion  is  very  slow  with  Bi-Sb  alloys  but  with 
Au-Ag  and  Au-Pt  alloys  the  thermal  treatment  is  quite  effective. 
All  alloys  whose  components  are  miscible  in  all  proportions 
do  not  show  the  kind  of  diagram  described  above,  that  is,  the 
steady  decrease  of  melting  and  crystallization  points  from  the 
melting  point  of  the  higher-melting  component  to  that  of  the 
lower.  Exactly  as  with  solutions  of  liquids,  whose  boiling  point 


¥UU 

600 
500 
400 

300 
268 

9nn 

Sb 

i 

B, 

X 

x 

/ 

X, 

i 
AI  x 

C.kx 

/ 

/ 

,/-' 

.^'  | 

1 
1 
1 

Bi 

622 


100 


FIG.  23. 


is  higher  or  lower  than  the  boiling  point  of  either  of  the  com- 
ponents, there  is  a  mixture  which  has  a  maximum  or  minimum 
boiling  point,  so  with  a  solid  solution  whose  melting  point  is 
higher  or  lower  than  that  of  the  components,  there  is  a  mixture 
of  maximum  or  minimum  melting  point.  The  liquid  mixture 
with  a  maximum  or  minimum  boiling  point,  boils  at  a  constant 
temperature,  the  composition  of  the  vapor  being  the  same  as 
that  of  the  liquid.  Similarly,  there  is  no  concentration  change 
in  the  melting  of  a  solid  solution  of  maximum  or  minimum  melt- 
ing point,  the  alloy  has  the  same  composition  in  the  solid  and 
liquid  state.  Alloys  of  this  kind  are  designated  as  coincident 
melting.  Schematic  diagrams  of  solid  solutions  with  a  maximum 


60 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


and  minimum  melting  point  are  shown  in  Fig.  240  and  b.  Out- 
side of  the  pure  components  only  the  mixture  of  maximum  or 
minimum  boiling  point  is  coincident  melting;  all  the  rest  melt 
over  a  temperature  range.  With  solid  solutions  of  the  type 
Sb-Bi  there  is  in  general  no  coincident  melting  alloy. 

The  miscibility  of  the  metals,  in  the  solid  state,  is  frequently 
only  partial,  for  example,  in  the  metal  pairs  Cu-Fe,  Cu-Pt,  and 
others.  Tammann  has  formulated  a  rule  concerning  these  cases, 
it  is  "  the  metal  with  the  higher  melting  point,  always  dis- 


Temp. 


Melt 


Solid  Solution 


Melt 


Solid  Solution 


Temp. 


FlG.  24. 

\ 

solves  more  of  the  metal  with  the  lower  melting  point  than  vice 
versa." 

The  crystallization  diagrams  of  alloys,  with  partial  mis- 
cibility of  the  components,  show  many  different  types  of  phe- 
nomena and  are  best  illustrated  by  examples.  The  gold-nickel 
alloys  (Fig.  25),  which  have  been  studied  by  Levin,  and  the 
cadmium-mercury  alloys,  which  have  been  studied  by  Bijl,  are 
such  examples.  In  the  first  case  the  occurrence  of  a  eutectic 
mixture  of  the  two  kinds  of  solid  solutions  is  interesting. 

Inter-metallic  Compounds. 

A  whole  series  of  phenomena  indicate  that  chemical  reaction 
takes  place  when  certain  metals  are  melted  together.  Among 


METALLIC   SOLUTIONS  AND  ALLOYS 


61 


J.UUV 

1500 
1400 
1300 
1200 

1100 
1064 

1000 

900 

NU1484 

/ 

' 

/ 

/ 

/ 

Au 

,    / 

\ 

\/A 

_j 

B 

c 

0     20    40    60    80    100 
FIG.  25. 


-50 


350 


• •  •  f i  j     t  )       C  A 

0    10   20   30   40   50   60  70  80  90  10o|cd 

100   90   80   70   60   50   40  30  20  10  0  -Hg 

FIG.  26. 


62 


THE   PHYSICAL  CHEMISTRY  OF  THE   METALS 


these  may  be  mentioned  the  strong  heat  evolution  when  sodium 
and  hot  mercury  are  mixed  and  also  when  copper  and  zinc  are 
mixed.  The  large  volume  difference  between  alloys  and  a  physi- 
cal mixture  of  their  components,  is  best  explained  by  such  a 
compound  formation. 

Alloys  frequently  show  properties  which  the  components  do 
not  possess.  The  alloys  of  antimony  and  manganese,  for  example, 
are  extraordinary  and  are  well  suited  to  lecture  demonstration 
of  the  change  of  components  by  melting.  If  a  mixture  of  equal 
molecular  proportions  of  antimony  and  manganese  are  fused,  in 


FIG.  27. — Powdered  Mn-Sb  Alloy  in  a  Magnetic  Field. 


a  hard  glass  tube,  a  reaction  takes  place,  and  after  cooling,  the 
powdered  alloy  behaves  like  iron  filings,  as  regards  a  magnet,  it 
being  possible  to  show  the  magnetic  lines  of  force  by  means  of  it. 
(See  Fig.  27.)  Neither  manganese  nor  antimony  as  elements 
are  magnetic,  but  manganese  compounds  frequently  are,  and  in 
all  the  magnetic  alloys  discovered  by  Heusler  *  the  magnetic 
character  is  connected  with  the  presence  of  manganese  com- 
pounds; for  example,  manganese  and  aluminum. 

*  Heusler,  verb.   Deutsch.   Physikalisch.   Gesselschaft,    1903,    219.     Stark   & 
Haupt,  Ibid.,  1903,  222.     Heusler,  Z.  Angew.  Chem.,  1904,  260. 


METALLIC  SOLUTIONS  AND  ALLOYS 


63 


Compounds  do  not  ordinarily  occur  between  .metals  that 
stand  near  each  other  in  the  periodic  system.  (Tammann).* 
They  are,  however,  no  rare  occurrence  and  many  metal  pairs 
form  several  compounds.  The  existence  of  the  following  com- 
pounds has  been  demonstrated:  Cu2Cd3,  Cu3Cd,  Cu3Al,  CuAl, 
Cu4Sn,  Cu3Sn,  CuSn,  Cu3Sb,  Cu2Sb,  Ag32n2,  AgZn,  Ag2Zn3, 
Ag2Zn5,  AgsAl,  Ag2Al,  Ag3Sb,  Au3Zn5,  AuZn8,  Au2Zn,  Au4Cd3, 
AuCd3,  Au4Al,  Au5Al2,  Au2Al,  AuAl,  AuAl2,  AuSn,  AuSn2, 
AuSn4,  AuSb2,  Au2Pb,  AuPb2,  Na4Sn,  Na2Sn,  Na4Sn3,  NaSn, 


A+  Eatectic 
A,  AB 


|  A  B  +  Eutectic 
A,  AB 


AB 
j-f- Eutectic 

I    B,  A  B 


B  -4-  Eutectic 
B,  AB 


Per  cent  B 


FIG.  28. 


NaSn2,  Na4Pb,  Na2Pb,  NaPb,  Na2Pb5,  Mg2Sn,  Mg2Pb,  SbAl, 
Sb2Zn3,  SbZn,  SbNa3,  SbNa,  Mg3Sb,  BiNa3,  BiNa,  ,BiMg3, 
Ni3Sn2. 

These  compounds  can  occur  as  separate  structure  constit- 
uents. They  can  also  form  solid  solutions  with  the  components, 
and  if  several  compounds  are  capable  of  existence,  dissociation 
and  recombination  come  into  consideration.  The  equilibrium 
diagram  may  be  very  complex,  if  all  these  phenomena  occur 
simultaneously  with  a  single  metal  pair.  It  is  not  possible  to 
discuss  all  known  cases  here,  so  we  will  limit  ourselves  to  the 
simplest  and  most  typical. 

*  Z.  anorg.  Chem.,  49,  113  (1906). 


64 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


If  a  stable  compound  of  the  two  components  is  formed  a  dia- 
gram like  Fig.  28  is  obtained.  A  maximum  is  seen  in  the  curve 
which  represents  the  melting  point  of  the  compound  A-B. 
This  melting  point  is  lowered  by  an  excess  of  A  or  B  and  curves 
are  obtained  which  represent  the  equilibrium  of  differently  com- 
posed melts  with  the  solid  compound;  these  solidification  curves 
intersect  on  the  one  hand  the  crystallization  curve  of  component 
A,  and  on  the  other  hand,  that  of  component  B.  Two  eutectic 
points  and  two  eutectic  mixtures  are  accordingly  possible,  for 


Per  cent  B    ^ 

FIG.  29. 

an  alloy  of  two  components,  which  form  one  compound.  If  the 
compound  A-B  decomposes,  before  it  melts,  an  equilibrium 
diagram  is  obtained  as  shown  in  Fig.  29.  Miscroscopically  the 
existence  of  two  eutectics  can  be  recognized,  but  only  one  eutectic 
point  can  be  observed  by  the  cooling  curve  method. 

The  diagram  for  the  occurrence  of  many  compounds  is  still 
more  complex,  but  the  principles  involved  are  the  same  as  with 
the  cases  discussed. 

Ternary  and  Quaternary  Alloys. 

In  practice,  alloys  are  frequently  met  which  are  composed  of 
more  than  two  metals.  We  shall  consider  here  only  the  bronzes 


METALLIC  SOLUTIONS  AND  ALLOYS  65 

and  the  bearing  metals  of  which  a  whole  series  of  different  com- 
position have  been  placed  on  the  market. 

The  easiest  to  treat  are  the  three  component  or  ternary 
alloys  containing  copper,  zinc,  antimony,  lead  and  tin.  The 
great  number  of  possible  structure  components  make  the  study 
of  these  complex  systems  quite  difficult.  But  to  give  an  idea  of 


FIG.  30— Ternary  Alloy  of  Sn,  Bi,  Pb.  White  Portion  Bismuth  crystals,  Lighter 
Portion  of  the  Ground  Mass,  Binary  (Bi-Sn)  Eutectic,  Dark  Portion  Ter- 
nary Eutectic. 

the  method  of  attack  on  ternary  systems,  the  combination  of  the 
three  metals  Zn,  Bi,  Pb,  which  is  not  complicated  by  the  forma- 
tion of  solid  solutions  or  compounds,  will  be  considered  rather 
closely.  It  has  been  investigated  by  Charpy.* 

We  will  first  consider  a  metallograph  of  this  kind  of  alloy 
(See  Fig.  30),  the  picture  shows  three  different  parts,  the  bright 
*  Contribution  a  1'etude  des  alliages,  121. 


66  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

large  surfaces  are  doubtless  the  first  component  to  be  precipitated 
and  can  be  considered  as  Bi.  The  crystals  are  embedded  in  a 
mass  which  has  the  appearance  of  a  eutectic,  but  this  ground  mass 
is  not  uniform,  and  light  and  dark  fields  can  be  distinguished 
from  each  other  in  it.  The  light  places  are  binary  eutectic, 
the  dark  places  ternary  eutectic.  The  binary  (Bi,  Zn)  pre- 
cipitates before  the  ternary. 

A  graphical  representation  of  the  solidification  relations  can- 
not be  made  in  the  simple  way  we  have  used  for  binary  systems. 
Representation  in  a  plane  is  only  possible,  when  the  number  of 
variables  is  two.  With  more  variables  a  space  model  must  be 
constructed.  We  may,  however,  represent  a  three-component 
system  at  constant  temperature.  The  points  no  longer  form  a 
line  but  a  plane.  Of  the  different  possible  representations,  that 
of  triangular  coordinate  axes  is  ordinarily  used.  The  side  of  an 
equilateral  triangle  is  divided  in  the  relation  a  :  b  :  c  of  the 
three  components,  a  parallel  to  the  side  AC  is  then  drawn 
through  the  end  point  of  a  and  this  made  equal  to  5.  In  this 
way  a  point  P  in  the  interior  of  the  triangle  is  obtained,  which  has 
notable  properties.  It  can  easily  be  proven,  that  a  perpendic- 
ular which  is  dropped  from  it  to  the  three  sides  AB,  BC,  AC 
are  in  the  same  proportion  as  the  coordinates  a  :  b  :  c,  and  that 
the  sides  of  the  three  small  equilateral  triangles  of  which  P  is 
the  mutual  vertex  are  equal  to  a  :  b  :  c  respectively.  This 
point  adapts  itself  excellently  for  the  graphic  representation 
of  the  composition  of  a  mixture  of  the  proportions  a  :  b  :  c. 
It  can  be  easily  seen  that  the  components  can  be  represented  by 
the  vertices  of  the  triangle  and  systems  of  two  components  along 
the  triangle  sides.  (Fig.  31.)  The  temperature  axis  is  now 
erected  in  a  vertical  plane,  and  a  three-sided  prism  is  obtained, 
in  which  the  equilibrium  diagram  in  space  is  placed.  The 
equilibrium  diagrams  of  the  two  component  systems  are  now 
represented  on  the  sides  of  the  prism.  In  the  case  of  the  system 
Zn,  Bi,  Pb,  these  are  simple  pairs  of  curves  with  a  eutectic  point. 
Experiment  shows  now  that  by  the  addition  of  a  third  metal  to 
a  binary  eutectic,  the  solidification  temperature  is  lowered. 
This  is  true  of  all  the  metal  pairs  and  there  are  three  space 


METALLIC  SOLUTIONS  AND  ALLOYS 


67 


curves,  which  represent  the  equilibrium  between  the  melt  and 
the  two  precipitated  metals.  The  equilibrium  of  the  melt 
with  any  one  solid  metal  is  represented  by  a  surface.  The  three 
space  curves  intersect  in  a  point,  a  ternary  eutectic  point,  at 
which  there  exists  a  simultaneous  equilibrium  between  the  melt 
and  the  three  different  precipitated  metals.  The  crystalliza- 
tion is  complete  at  the  ternary  eutectic  point. 


FIG.  32. 


The  relations  between  the  binary  and  ternary  eutectic  points 
and  the  composition  of  the  different  eutectics  are  shown  in  the 
following  table: 


Components. 

Eutectic 
Temp. 

Atomic 
Proportions. 

Binary  eutectics  

(Sn     Pb 
\  Sn             Bi 

183 
175 

77  :  23 
58             :  42 

Ternary  eutectics 

I         Pb    Bi 
Sn    Pb     Bi 

125 
06 

56.2  :43-8 

24  I  4.6  I  28 

Ternary  eutectic  with  addition 
of  a  fourth  metal  

Sn    Pb    BiCd 

65   c 

Woods  metal 

68  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

A  quick  insight  into  the  equilibrium  changes  in  a  ternary  alloy 
can  be  obtained  from  a  projection  of  the  space  diagram  on  the 
basal  plane  and  connecting  the  points  representing  combinations 
which  crystallize  at  the  same  temperature.  In  this  way  a  mul- 
titude of  isotherms  is  obtained 
which  show  the  height  above 
the  basal  plane,  just  as  the 
contour  lines  on  topographical 
maps.  Such  a  projection  is 
shown  in  Fig.  33. 

The  addition  of    a    fourth 
metal  sometimes  causes  a   de- 
pression of   the   melting   point 
below  that  of  the  eutectic  point 
FIG.  33.  of  the  ternary  alloy  as,  for  ex- 

ample, shown  for  Woods  metal 

in  the  preceding  table.  The  different  low  melting  alloys  are  of 
this  type.  Some  of  these  alloys  melt  in  hot  water  as,  for 
example,  the  metals  of  Newton,  Lichtenberg  and  Wood  and  have 
found  practical  use  in  many  ways. 

Relations   of   Mechanical   and   Physical   Properties   to   Alloy 
Structure. 

The  mechanical  and  physical  properties  of  alloys  are  depend- 
ent on  the  structure  and  this  fact  has  given  practical  importance 
to  metallography  for  the  testing  of  materials. 

It  is  clear,  that  an  alloy  with  the  most  homogenous  possible 
structure,  which  has  the  same  components  as  another,  con- 
taining interstratified  large  crystals,  will  excel  in  tensile  and 
compressive  strength.  Where  the  size  is  important  the  use  of 
eutectic  alloys  or  alloys  that  consist  of  a  homogenous  solid  solu- 
tion is  to  be  recommended.  The  elastic  limit  seems  always  to  be 
a  maximum  in  the  eutectic  alloys  so  far  as  the  few  investigations 
allow  a  conclusion.  (Ssaposhnikow.)  It  is  still  premature  to 
make  any  general  statements  due  to  the  great  difference  which 
the  several  metals  show  in  their  properties. 

The  question  of  the  relation  of  hardness  to  structure  has  been 


METALLIC   SOLUTIONS  AND  ALLOYS 


69 


recently  treated  by  Russian  investigators.  Ssaposhnikow  *  and 
his  co-workers  have  established  that  with  normal  alloys  of  two 
components  the  eutectic  possesses  the  maximum  hardness.  The 
results  with  the  zinc-cadmium  alloys  are  given  in  the  following 
table: 


Zinc, 
Per  Cent. 

Hardness. 

Zinc, 
Per  Cent. 

Hardness. 

Zinc, 
Per  Cent. 

Hardness. 

0 

15-9 

24.4 

32.2 

79-9 

43-0 

IO 

31-5 

30 

27.2 

80 

52.0 

12.5 

33-o 

40 

31.2 

81 

46 

17.2 

36-8 

SO 

35-o 

82.2 

39-0 

17-6 

38.8 

60 

34-o 

90 

39-o 

20 

34-5 

70 

34-5 

100 

35-0 

The  second  hardness  maximum  in  the  alloy  of  80  per  cent  Zn 
is  not  clearly  understood.  (The  pressure  in  Kg/sq.cm.  neces- 
sary to  press  a  steel  ball  of  10  mm.  diameter  into  the  alloy  serves 
as  a  measure  of  the  hardness). 

Hardness  maximums  are  also  shown  by  alloys  of  the  com- 
ponents Pb-Sn,  Al-Zn. 

Solid  solutions  are  usually  somewhat  harder  than  their  com- 
ponents and  alloy  systems,  which  are  made  up  of  a  continuous 
series  of  solid  solutions  usually  have  a  hardness  maximum. 

Bearing  Metals. 

The  hardness  relations  of  the  structure  components  play  an 
important  role  in  the  so-called  bearing  metals.  The  theoretical 
treatment  of  this  subject  is  due  to  Charpy.f  A  good  bearing 
metal  must  consist  of  a  plastic  ground  mass  in  which  hard  crystals 
are  embedded  in  a  uniform  way.  Such  a  structure  offers  the 
best  security  that  the  bearing  itself  will  be  continually  adjusted 
to  the  form  of  the  axle  without  the  material  becoming  attached 
to  the  axle.  Further  there  occurs  a  continual  yield  of  the  bearing 
material,  and  the  abrasion  coefficient  of  it  must,  therefore,  be 
high  so  that  the  bearing  is  not  easily  heated.  An  entirely  hard 

*  Chem.  Zentr.,  1908,  I,  in. 

t  Contribution  a  1'etude  des  alliages,  121. 


70  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

material  will  not  cling  to  the  axle,  but  it  does  not  give  sufficiently 
to  the  pressure.  All  these  conditions  are  met  by  an  alloy  of 
the  above-described  structure,  since  by  a  change  of  pressure  the 
hard  crystals  in  the  yielding  mass  are  immediately  pressed  out. 
The  structure  of  a  bearing  metal  is  shown  in  Fig.  34.  It  repre- 
sents an  alloy  of  the  components  Sn,  Sb,  Pb,  the  bright  crystals 


FIG.  34. — Bearing  Metal  Alloy  of  Tin,  Antimony,  and  Lead. 

are  of  the  compound  SbSn  and  the  dark  ground  mass  is  ternary 
eutectic. 

Density  of  Alloys. 

The  relations  of  certain  other  constants  of  alloys  to  the 
structure  is  known  with  considerable  accuracy,  viz.,  the  density, 
the  electrical  potential  and  especially  the  electrical  resistance. 

In  many  cases  the  density  of  an  alloy  can  be  calculated  from 
the  density  of  its  components,  by  the  rule  of  mixtures.  The 
reciprocal  value  of  the  density,  the  specific  volume,  can  be  fre- 
quently represented,  as  Maey  *  has  shown,  by  a  linear  equation 
of  the  form 

VL  =  VA-b.p.B 

*  Z.  Phys.  Chem.  38,  299  (1901). 


METALLIC   SOLUTIONS  AND  ALLOYS 


71 


Where  VL  is  the  specific  volume  of  the  alloy,  VA  that  of  the  com- 
ponent A ,  pB  the  percentage  of  B  and  b  a  constant.  In  the  fol- 
lowing table  are  given  the  values  for  VA  and  b  which  are  neces- 
sary to  calculate  VL. 


COMPONENTS. 

VA. 

&. 

Kind  of 
Alloy. 

A. 

B. 

Lead 

Antimony 

0.08791 
0.08791 
0.08791 
o.  0881  i 

0-IIS54 
0.05191 
0.05191 
0.05191 
0.04461 
0.07368 
0.07366 
o  .  0948 

0-0955 
0.10181 
o.  10181 
o.  10181 
0.13710 
0.13710 

0.0006106 
0.0002763 
0.000076 
0.00049 
o.  (3002156 
o  .  000605 
o  .  000605 
0.000852 
0.0000191 
0.0001422 

0.0006345 

0.000169 
0.000063 
0.0004715 
0.0001373 

0.000353 

0.0001187 
0.00004 

K 
K 
K 
K 
K 
M(p) 
M(l) 

K 

K 

M(/) 
K 

VM 

K 

Lead      

Cadmium  

Lead 

Silver 

Lead  

Tin  

Cadmium  

Tin  

Gold 

CoDoer 

Gold  
Gold              .... 

Silver  

Tin     

Iridium 

Platinum 

Mercury  

Lead      

Mercury   .  .  . 

Tin 

Silver            . 

Copper 

Silver       

Bismuth        

Bismuth 

Antimony  
Cadmium  

Bismuth  

Bismuth,  

Zinc  

Tin 

Antimony  
Zinc 

Tin 

K  means  simultaneous  deposition  of  the  two  metals,  M  forma- 
tion of  solid  solutions,  /  total  miscibility,  p  partial  miscibility, 
V  formation  of  compounds. 

No  volume  change  occurs  in  the  formation  of  solid  solutions 
in  these  alloys.  There  is,  however,  a  contraction,  if  the  com- 
ponents combine  with  each  other,  hence  a  minimum  of  specific 
volume  occurs  in  alloys  of  Sb  and  Fe  and  Cu  and  Sn  when  their 
composition  expresses  that  of  a  compound,  as  FeSb  or  CusSn 
(Fig.  35).  A  large  contraction  cannot  always  be  explained  by  the 
formation  of  a  compound,  for  example,  with  tin  and  silver  the 
minimum  volume  corresponds  to  a  composition  of  29.2  per  cent 
tin  and  with  lead  and  bismuth  the  maximum  contraction  occurs 
with  40-50  per  cent  bismuth  while,  according  to  the  equilibrium 


72 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


diagrams  the  alloys  of  this  composition  must  consist  of  normal 
conglomerates  of  their  pure  components  (Roozeboom). 

Potential  of  Alloys. 

Of  the  electrical  magnitudes  which  are  capable  of  giving  an 
insight  into  the  nature  of  structure  components  we  shall  first 
consider  potential,  which  has  been  recently  investigated  by  the 
Russian  investigator  Puschin.*  He  measured  the  potential 
difference  between  the  binary  alloys  and  their  least  noble  com- 


8.5 
8.0 
7.5 
7.0 


0 
100 


20 


30 
70 


40 
GO 


50 
50 


70 
30 


20 


00 
10 


FIG.  35. 


9.0 
8.5 
8.0 
7.5 
7.0 


100  ^Sn 
o  "Cu 


ponents  using  a  salt  of  the  latter  as  an  electrolyte.  The  follow- 
ing rules  have  been  formulated  for  the  relation  of  potential  to 
composition. 

(1)  If  the  alloy  is  a  conglomerate  of  components  the  poten- 
tial difference  is  independent  of  the  composition. 

(2)  With  the  formation  of  a  continuous  series  of  solid  solu- 
tions the  potential  difference  rises  with  increasing  content  of  the 
noble  metal. 

(3)  With  the  formation  of  compounds  sudden  changes  occur 
in  the  potential  difference. 

*  Chem.  Zentr.,  1907, 1,  1724;  II,  1315,  2026;  1908, 1,  108. 


METALLIC  SOLUTIONS  AND  ALLOYS 


73 


Electrical  Resistance  of  Alloys. 

The  electrical  resistance  of  an  alloy  frequently  shows  import- 
ant imprints  of  its  character  and  from  the  relation  of  the  con- 


0    10    20    30    40   50    60    70    80   90   100  ^Cd 
100   90   80    70   60   50    40    30    20   10    0 


FIG.  36. 


Ag- 


>Au 


0    10    20    30    40    90    60    70    80    90   100$  Ag 
100   90    80    70    60    50    40    30    20    10    0  "All 

FlG.  37. 

ductivity  to  the  composition,   conclusions  can  frequently  be 
drawn,  regarding  the  structure. 


74 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


There  is  a  great  deal  of  experimental  material  concerning 
the  conductivity  of  alloys.  The  investigations  of  Mathiessen  * 
are  of  especial  value.  By  comparison  of  his  results  with  the 
metallographic  investigations  of  Guertler  f  and  the  earlier  inves- 
tigations of  LeChatelier  {  the  following  general  rules  have 
developed. 

(i)  Alloys,  which  are  purely  mixtures  of  their  components, 
have  a  conductivity  which  is  additively  made  up  of  the  conduc- 
tivity of  their  components. 

Copper  Antimony 


14 

13- 

12 

11- 

10- 
9- 
8- 
7- 
6- 
5- 


Cu2Sb 


-13 


12 


20  40  60 

FIG.  38. 


80 


fcSb 


(2)  Alloys  which  consist  of  solid  solutions  show  an  important 
lowering  of  conductivity. 

(3)  The  occurrence  of  compounds  can  frequently  be  recog- 
nized by  the  occurrence  of  peaks  in  the  composition-conductivity 
diagram. 

The  alloy  of  zinc  and  cadmium  is  an  example  of  a  mixture, 
(see  Fig.  36),  while  the  pair  silver,  gold  is  an  example  of  the 
solid  solutions  (see  Fig.  37).  The  alloys  of  copper-antimony 
show  the  effect  of  compound  formation  (see  Fig.  38). 

*  Pogg.  Ann.,  110,  222  (1860). 

t  J.  Inst.  Metals,  No.  2,  1911,  VI,  p.  135. 

t  Rev.  gen.  Sci.,  30,  June,  1895. 


METALLIC   SOLUTIONS  AND  ALLOYS 


75 


The  temperature  coefficient  of  the  conductivity  shows  a 
similar  regularity  which  has  also  been  noted  by  Guertler.* 
With  conglomerates,  the  temperature  coefficient  is  equal  to  that 
of  the  pure  metals,  and  the  resistance  of  all  this  kind  of  alloys  van- 
ishes as  the  metals  reach  absolute  zero.  The  alloys  which  con- 
sist of  solid  solutions,  show  in  comparison  to  their  components, 
a  very  small  temperature  coefficient,  and  their  resistance  does  not 

26 


AU 


FIG.  39. 


vanish  at  absolute  zero.  In  general  there  is  a  proportionality  be- 
tween the  conductivity  of  alloys  and  their  temperature  coefficient. 
The  form  of  the  curve  expressing  the  temperature  coefficient  as 
a  function  of  the  volume  composition,  has,  without  exception,  the 
same  form  as  the  curve  for  conductivity.  These  facts  can  be 
recognized  by  a  comparison  of  Fig.  39  with  the  earlier  figures. 

*Z.  anorg.  Chem.,  51,  397  (1906);   54,  58  (1907);   Z.  Elektrochem.,  13,  441 
(1907). 


76 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


The  electrical  relations  of  solid  solutions  are  not  only  of 
interest  for  their  bearing  on  metallography,  but  also  for  their 
practical  importance.  The  valuable  resistance  metals  con- 
stantan  and  manganin  are  solid  solutions.  We  will,  therefore, 
look  a  little  farther  into  the  theoretical  problems  which  they 
present,  and  see  if  we  can  discover  what  factors  condition  the 
strong  influence  of  small  additions  of  a  second  metal  on  the  con- 
ductivity. 

As  the  following  table  shows,  considerable  decrease  in  the 
conductivity  of  metallic  copper  is  caused  by  the  addition  of  2 
per  cent  of  other  material. 


Thermo-electric 

force  of  the  dis- 

Copper +2%  Metal 

Atomic  Weight 
of  addition 

Electrical  Cond. 
Xio* 

solved  metal 
against  copper. 

Temp.  diff.  100°. 

Millivolt 

Cu.  Dure 

62.1 

WtftJ    ^/HAV* 

Ag 

108 

55-i 

—  o.oi 

Au 

197.2 

47-5 

+0.02 

Zn 

65.4 

45 

+0.03 

Sn 

118.5 

26.5 

—  0.30 

Ni 

58.5 

26.0 

-2.15 

Al 

27.1 

17 

—  0.32 

Mn 

55 

12 

Fe 

56 

13 

+  I.IQ 

Especially  large  lowering  is  obtained  with  alloys  of  copper 
and  nickel,  whose  components  are  miscible  in  all  proportions, 
this  is  shown  in  diagram  40  and  the  next  table. 


Metal 

Conductivity  Xio« 

Ratio 

Copper.  .  . 

C7.A 

IOO 

Nickel    .  . 

13*30 

23.  3 

Constantan, 

6oCu,  4oNi.                      

2.04 

•2.  er 

The  smallness  of  the  temperature  coefficient,  is  brought  out 
by  reference  to  the  next  table.  It  is  even  possible  to  obtain  an 
alloy  whose  resistance  is  independent  of  the  temperature.  This 
kind  of  a  resistance  metal,  is  of  great  practical  importance,  for 


METALLIC   SOLUTIONS   AND  ALLOYS 


77 


the  preparation  of  precision  resistances,  as  they  are  uninfluenced 
by  the  temperature. 


Mets 

il 

Temp.  Coeff.  of 
resistance  at  18° 

Metal 

Temp.  Coeff.  of 
resistance  at  18° 

Cu  1  00% 

-{-O  00428 

Au  1  00% 

-j-o  00368 

Cu  80%,  Ni 

20%.. 

-(-0.000262 

Au  90%,  Ag  10%  

+o  00124 

Cu  u%  Ni 

46% 

-|-o  oooo 

Au66%,  Ag^% 

~f~o  00067 

Ni  1  00% 

-4-Q  OO^CK 

Ag  100% 

-j-o  00400 

Constantan  and  manganin  which  show  these  valuable  prop- 
erties are  solid  homogenous  solutions. 

It  is  of  interest  in  this  connection,  that  there  is  also  an 
elementary  metal,  which  possesses  an  extraordinarily  low  tem- 
perature coefficient,  of  the  same  order  of  magnitude,  as  that  of 
many  alloys,  namely,  0.0008.  It  is  mercury  in  the  liquid  state. 
Liebenow  has  drawn  the  conclusion  from  this,  that  mercury  is  a 
solution,  which  contains  in  it  a  second  molecular  species.  It  is 
assumed  for  example,  that  in  it  outside  of  the  mercury  molecules 
Hg,  a  second  molecule  Hg2  is  present  in  equilibrium  with  the 
first.  In  the  solid  state  mercury  behaves  normally. 

The  question  of  the  remarkable  conductivity  lowering  of 
alloys  is  often  raised  but  it  has  not  been  answered  in  every  way. 
Lord  Rayleigh  and  later  Liebenow  have  attempted  to  explain 
the  phenomena  by  the  Peltier  effect,  that  is,  the  thermo-current 
causes  an  increase  in  resistance.  However,  at  the  time  of  the 
presenting  of  this  theory,  we  were  not  so  well-informed  on  the 
structure  relations  of  alloys,  and  in  the  light  of  our  present 
knowledge  it  is  quite  inadequate. 

The  conductivity  lowering  can  be  explained,  however,  in 
terms  of  the  electron  theory.  It  is  first  necessary  to  find  whether 
the  heat  conductivity  shows  a  similar  lowering  for  these  alloys, 
and  to  ascertain  whether  the  law  of  Wiedemann  and  Franz 
holds  with  certainty.  With  the  Bi-Pb  and  Bi-Sn  alloys  in  which 
the  ability  to  form  solid  solutions  is  confined  to  narrow  limits  a 
parallelism  has  been  established  between  the  electrical  and  heat 
conductivity  (F.  A.  Schulze).*  Compare  the  following  table 
with  Fig.  41. 

*  Habilitationschrift,  Marburg,  1902. 


78 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


From  these  measurements  it  follows  that  the  ratio  of  the  two 
conductivities  of  the  alloys  is  somewhat  greater  than  for  pure 
metals.  The  same  results  have  been  obtained  by  differently 
carried  out  investigations  on  the  Ni-Cu  series. 


Alloy. 

Volume,  Per  Cent. 

Heat  Con- 
ductivity. 

X 

Electrical 
Conduct- 
ivity. 

I05 

X 

a 

Bi 

Pb 

IOO 

0.019 

0.830 

230 

Bismuth-Lead  < 

99-57 
98.44 

97-31 

0-43 
1.56 
2.69 

0.0188 
0.0119 
O.OII7 

0.766 
0.468 
0.444 

245 
256 
263 

96.47 

IOO 

99-54 

3-53 

0.0129 

0.0190 
0.0140 

0.514 

0.830 
0-595 

251 

230 
235 

0.46 

Bismuth-Tin  

99-05 

0-95 

0.0126 

0.506 

248 

97-13 

2.87 

O.OIIO 

0.448 

245 

90.26 

9-74 

0.0126 

0.488 

258 

Metal. 

X 

a 

Observer. 

Cu,  pure  

665 

Jaeger  and  Diesselhorst  * 

Ni,  pure      

669 

Jaeger  and  Diesselhorst 

60  Cu,  40  Ni                     

1106 

Jaeger  and  Diesselhorst 

<4  Cu  4.6  Ni 

90  1 

Gruneisen  f 

84  Cu  4  Ni   12  Mn 

014 

Jaeger  and  Diesselhorst 

*  Loc.  cit. 

t  Ann.  Phys.,  3,  71  (1900). 

Copper  and  nickel  are  miscible  in  all  proportions  and  accord- 
ingly the  curve  which  represents  the  relations  of  the  conductivity 
quotient  to  the  composition  possesses  a  maximum. 

The  increase  of  the  conductivity  quotient  shows  that  the 
dissolved  metal  lowers  the  heat  conductivity  relatively  less  than 
the  electrical.  How  is  this  fact  to  be  explained?  If  we  take 
as  our  basis  the  Drude  electron  theory  we  see  that  the  heat  con- 
ductivity is  concerned  with  heavier  particles"  than  the  con- 
duction of  electricity.  As  diffusible  particles  we  have  now  in 
these  solid  solutions,  beside  the  electrons,  the  dissolved  mole- 


METALLIC   SOLUTIONS  AND  ALLOYS 


79 


cules  of  the  second  metal.  These  will  fulfill  the  requirements 
for  heat  conductivity  but  will  not  take  part  in  the  electrical  con- 
ductivity. They  are  to  be  considered  as  electrically  neutral 
and  undissociated. 

Now,  however,  the  electrical  conductivity  decreases.  Such  a 
decrease  can,  according  to  our  previous  considerations,  result 
in  two  ways;  in  decreasing  the  number  of  electrons  or  in  the 

,20  10" 


10 


Copper  Nickel 


V 

£ 

3 
2 
1 
0 


1 10 


1.00 


0.90 


.§0.80 
4    1 

,  I" 

K0.60 


0.50 


0.40 


i-Sn 


Z. 


19 


15 


11 


20        40        60        80       100 
Volume  per  cent  Nickel 

FlG.  40. 


95    96    97    98    99   100 
Volume  per  cent  Bi 

FlG.  41. 


resistance  which  they  encounter  in  their  motion  under  the  influ- 
ence of  the  e.m.f.  becoming  greater  due  to  the  increase  of  internal 
friction.  Electrons  and  molecules  both  diffuse  in  the  solid  metal- 
lic solutions;  we  have  accordingly  to  do  with  the  diffusion  of  a 
mixture.  The  kinetic  theory  allows  us  to  draw  certain  con- 
clusions concerning  the  friction  in  a  gas  mixture,  which  have 
been  found  to  agree  with  the  experimental  facts.  The  vis- 
cosity of  a  gas  is  increased  by  the  addition  of  a  second  gas  even 
though  the  second  gas  possesses  a  smaller  coefficient  than  the 


80  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

first.  (This  is  true  only  in  certain  cases,  and  is  not  true  in  case 
of  oxygen-hydrogen,  nitrogen-hydrogen,  hydrogen-helium  or 
oxygen-nitrogen,  but  is  true  for  carbon  dioxide-hydrogen  and 
helium-argon).  Graham,  and  later  Puluj,  both  obtained  the 
remarkable,  and  on  first  thought  improbable,  result,  that  the 
internal  friction  of  C02  is  not  lowered  but  raised  by  the  addition 
of  a  small  amount  of  hydrogen.  This  increase  reaches  a  maxi- 
mum with  27  per  cent  H2  and  73  per  cent  C02  at  room  tem- 
perature. 

Now,  if  we  carry  over  the  kinetic  theory  to  a  mixture  of 
electrons  and  molecules,  it  follows  that  the  internal  friction  of 
the  electrons  increases  with  the  addition  of  molecules  and, 
therefore,  the  electrical  conductivity  decreases.  The  lowering 
of  the  conductivity  can  be  explained  then  even  if  the  number 
of  electrons  does  not  decrease. 

A  decrease  of  the  electron  number  in  the  alloys  could  be  ascer- 
tained as  we  have  previously  seen  by  a  change  in  the  optical 
properties  (compare  p.  31),  especially  by  a  lowering  of  the 
reflection  constant.  There  is,  however,  no  experimental  data 
on  the  optical  constants  of  alloys  as  compared  with  their  com- 
ponents. 

Thermo-electric  Force  of  Alloys. 

The  comparison  of  the  thermo-electric  forces  of  the  alloys  and 
their  components  gives  us  a  second  means,  but  the  experimental 
material  is  also  scarce  in  this  field.  However,  there  is  enough 
for  a  preliminary  consideration.  Jaeger  and  Diesselhorst  have 
determined  the  thermo-electric  force  of  constantan  and  man- 
ganin  against  copper.  If  one  junction  is  held  at  o°  and  the  other 
at  100°  there  results  the  following  electromotive-force. 

Constan tan-copper         —  3440  micro  volts 
Manganin  copper          -f-   5 70  micro  volts 

According  to  Drude  the  thermo-electric  force  is  connected 
with  the  electron  number  N  of  the  two  metals  by  the  equation 
(v.  p.  32). 

Ncu 


(micro- volts)  =  i .71  (Tz  —  T\ )  loge 


AT 

A  alloy 


METALLIC   SOLUTIONS  AND  ALLOYS  81 

The  positive  pole  at  the  warm  junction  is  the  electron  richer 
metal.  It  follows,  therefore,  that  manganin  has  a  higher  and 
constantan  a  lower  concentration  than  copper.  From  the  given 
results  the  ratio  of  the  electron  numbers  is  calculated. 

Ncu 

*r-      -  =  1-223, 


•i  V  constantan 

Ncu = 

manganin 


N 


O.97O 


If  the  electron  number  of  copper  is  now  placed  arbitrarily  at  100 
it  follows  that 

Ncu  '  N  Constantan  =  TOO  :  81, 

• 

Ncu  -  N  Manganin    =  100  :  103. 

We  see,  therefore,  that  the  electron  concentration  of  an  alloy 
is  not  necessarily  smaller  than  that  of  the  pure  metals.  The 
ratio  of  the  electrical  conductivity  between  copper  and  its  two 
alloys  is 

acu  '  o-  Constantan  =  100  :  3.55, 

<rCu  •  v  Manganin    =100  :  4.16. 

A  comparison  of  these  ratios  with  the  previous  ones  shows  at 
once  that  of  the  two  factors  which  can  accomplish  a  change  in 
conductivity  the  electron  number  and  the  frictional  resistance, 
only  the  latter  really  comes  into  consideration. 

To  have  come  to  this  conclusion  shows  the  value  of  Drude's 
kinetic  electron  theory  of  metals. 


CHAPTER  III 

ALLOYS  OF  METALS  WITH  CARBIDES,  OXIDES  AND 
SULFIDES.  IRON  AND  STEEL,  MATTES,  PHASE 
RULE. 

Compounds  with  Metallic  Properties. 

A  series  of  compounds  of  metals  with  non-metals  exist  which 
possess  certain  properties  characteristic  of  metals.  The  nat- 
urally occurring  ores,  galena,  chalcopyrite  and  pyrite,  belong  to 
this  series  and  are  colloquially  called  "  Fools  Gold."  They 
are  characterized  by  their  strong  light  absorption,  their  light 
reflection,  and  their  luster  as  well  as  by  a  metallic  conduction 
of  electricity.  These  metallic  properties  are  not  restricted  to  the 
sulfides  but  also  occur  with  certain  oxides,  phosphides,  selenides 
and  carbides. 

There  are  certain  regularities  in  the  existence  of  compounds 
possessing  metallic  properties.  We  do  not,  for  example,  meet 
them  in  the  compounds  of  the  alkali  and  alkali  earth  metals, 
these  usually  have  a  salt-like  character.  The  tendency  of  these 
metals  to  go  over  into  the  ionic  state  with  water  is  especially 
large.  With  the  compounds  of  these  metals  with,  for  example, 
phosphorus  or  carbon,  there  is  a  decomposition  with  water, 
calcium  phosphide  gives  phosphine,  calcium  carbide,  acetylene, 
leaving  a  solution  of  calcium  hydroxide.  All  easily  ionizable 
metals  give  salt-like  or  easily  decomposable  compounds. 

The  possibility  for  the  occurrence  of  metal-like  compounds  is 
the  greatest  with  metals  of  small  ionizing  tendency,  especially 
when  the  non-metallic  part  also  shows  a  small  tendency  to  ioni- 
zation.  A  chloride  of  metallic  luster  is  scarcely  probable  but 
silver  iodide  possesses,  under  certain  conditions  of  lighting,  a 
metallic  conductivity.  Metallic  properties  are  sometimes  met 

82 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  83 

in  the  oxides,  as  examples  may  be  given  here  the  oxides  of  the 
platinum  metals  as  well  as  copper  oxide  Cii20. 

The  dependence  of  the  properties  on  the  nature  of  the  metal 
is  best  surveyed  with  the  sulfides.  The  alkaline  sulfides  have  an 
entirely  salt-like  character,  the  sulfides  of  the  heavy  metals  on 
the  other  hand,  PbS  and  Ag2S  are  types  for  the  occurrence  of 
metallic  properties  in  metallic  compounds.  Between  these  are 
the  sulfides  of  metals  of  somewhat  greater  ionization  tendency 
as,  for  example,  ZnS  which  forms  crystals  which  are  transparent 
in  thin  layers  but  which  are  still  insoluble  in  water. 

Since  compounds  of  metallic  properties  do  exist,  it  is  not 
surprising  that  they  are  miscible  with  and  soluble  in  the  metals. 
These  alloys  of  metals  with  compounds  are  of  great  importance 
in  metallurgy.  The  alloys,  for  example,  of  copper  with  phos- 
phorus and  silicon  have  valuable  mechanical  properties.  The 
presence  of  phosphides  in  iron  on  the  other  hand  is  undesirable 
since  the  solidity  is  adversely  influenced.  We  will  now  con- 
sider the  alloys  of  metals  with  metallic  compounds  especially 
the  very  important  series  of  iron  and  carbon  or  rather  iron  and 
iron  carbide. 


The  Iron-carbon  Alloys. 

The  great  influence  that  a  content  of  carbon  exerts  on  the 
properties  of  iron  has  been  known  for  a  long  time.  The  hard- 
ness is  known  to  stand  in  direct  relation  to  it.  A  chemical 
investigation  of  the  varieties  of  iron  has  shown  that  the  physical 
properties  are  not  solely  determined  by  the  chemical  composition 
and  that  steels  of  the  same  elementary  composition  frequently 
show  extraordinary  differences  in  properties.  That  the  carbon 
occurs  in  different  forms  can  be  shown  by  dissolving  the  alloys  in 
dilute  acid;  in  some  cases  the  residue  is  graphite  or  amorphous 
carbon  and  in  other  cases  the  alloy  dissolves  completely  in  acids, 
with  the  evolution  of  hydrogen  containing  large  amounts  of 
hydrocarbons  which  shows  that  the  carbon  was  in  the  combined 
form  as  carbide.  With  special  precautions  the  carbide,  which  is 
more  slowly  attacked  than  the  iron,  may  be  isolated.  The  inves- 


84  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

tigations  of  Rathke  *  and  of  Foerster  f  have  shown  that  this 
carbide,  which  is  designated  as  cementite,  has  the  formula  FeaC. 

The  correct  ratio  for  each  of  the  great  variety  of  properties, 
which  can  be  obtained  by  carbon  content,  can  only  be  deter- 
mined by  a  complete  knowledge  of  the  solidification  process 
of  the  mixture  and  the  changes  which  it  undergoes  in  the  solid 
state.  The  first  attempt  to  establish  an  equilibrium  diagram  for 
the  system  iron-carbon  was  made  in  the  year  1897  by  Sir  William 
Roberts- Austin  {  on  the  basis  of  the  studies  of  Osmond  and 
himself  on  the  crystallization  and  recalescence  points  of  iron. 
An  essential  advance  was  made  in  this  direction  by  the  work  of 
Bakuis  Roozeboom  §  in  the  year  1900  who,  simultaneously  with 
LeChatelier,  applied  the  phase  rule,  as  a  critical  expedient  for 
the  investigation  of  complex  systems. 

A  large  number  of  works  treating  this  subject  have  appeared 
in  recent  years.  The  works  of  Heyn  ||  Stansfield,  Benedicks, If 
Wust  **  and  others  have  cleared  up  dark  points  and  removed 
actual  contradictions,  but  in  such  a  complexity  of  phenomena 
there  is  much  that  is  still  unexplained,  ff 

If  we  consider  the  two  components,  iron  and  carbon,  it  is  seen 
that  both  are  known  in  different  modifications.  The  metal 
solidifies  at  1530°  and  there  are  no  fewer  than  three  polymorphic 
iron  modifications  a,  /3,  and  7  iron  with  the  transition  points  at 
768°  and  898°.  Graphite  and  amorphous  carbon  are  the  only 
forms  of  carbon  that  occur  in  these  alloys.  The  transition  point 
between  the  carbon  modifications  is  not  known.  It  is  only 
known  that  graphite  is  the  more  stable  of  the  two  and  that  the 
transition  takes  place  only  in  the  sense  of  amorphous  carbon  to 

*Ann.,  260,  333  (1890). 

t  Ber.,  29,  2991  (1896). 

J  Fifth  Report  of  the  Alloys  Research  Committee:    Proc.  Inst.  Mech.  Eng., 
1899,  Vol.  2,  3,  5. 

§  Z.  Physik.  Chem.,  34,  437  (1900). 

||  Z.  Elektrochem.,  10,  491  (1904). 

If  Mettalurgie,  3,  393  (1906). 
**  Dissertation,  Aachen  (1900). 

tt  Recent  papers  of  interest  on  the  iron-carbon  diagram  are,  Carpenter,  J.  Iron 
and  Steel  Inst.,  No.  i,  1913;  Benedicks,  ibid.,  No.  2,  1912;  Rosenhain  and  Hum- 
phrey, Proc.  Roy.  Soc.,  1909,  A83,  200. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  85 

graphite.  It  is  not  contradictory  to  this  that  the  former  pos- 
sesses an  extraordinary  stability  at  high  temperatures.  The 
two  elements  combine  to  form  a  carbide  which  as  we  have  noted 
is  cementite  FesC.  The  combination  is  best  effected  by  dis- 
solving the  carbon  in  liquid  iron.  The  condition  in  such  a  liquid 
melt  is  probably  an  equilibrium  between  iron,  carbon  and  car- 
bide. The  pure  cementite  is  not  a  stable  form  in  the  solid  state 
in  spite  of  its  comparatively  great  permanence,  since  it  is  decom- 
osed  by  long-continued  heating  at  1000°  into  a  solid  solution  of 
carbide  in  iron,  and  carbon,  which  appears  as  temper  carbon 
(amorphous).  The  dissociation  of  the  carbide  into  iron  and 
graphite  occurs  only  in  the  liquid  state  or  in  any  case  only  in  the 
vicinity  of  the  melting  point  (Wust).* 

If  the  cooling  and  crystallization  of  a  molten  iron  carbon  alloy 
does  not  go  too  slowly  the  precipitation  of  elementary  carbon  is 
entirely  avoided,  the  total  carbon  being  found  in  the  form  of 
carbide. 

If  we  would  now  survey  the  crystallization  phenomena  of  a 
liquid  iron  carbon  alloy,  it  is  expedient  to  first  consider  the  meta- 
stable  systems  that  result  from  the  quick  cooling  crystallization 
and  then  to  treat  the  solidification  with  the  precipitation  of 
elementary  carbon.  In  this  we  follow  C.  Benedicks. 

The  Crystallization  of  the  System  Iron-cementite. 

We  will  begin  with  a  melt  very  rich  in  carbon  and  allow  it  to 
crystallize.  It  precipitates  first  cementite  needles  and  the  melt 
becomes  rich  with  the  other  components  as  the  temperature 
sinks  till  finally  it  reaches  the  eutectic  point  at  which  complete 
solidification  takes  place.  This  eutectic  point  is  at  1130°.  The 
content  of  the  eutectic  mixture  is  represented  by  4.2  per  cent  C. 
and  62.9  per  cent  carbide.  If  the  carbon  content  of  the  melt  is 
smaller  than  that  of  the  eutectic  alloy  the  other  component 
crystallizes  out  first.  The  precipitation  product  is,  however, 
not  pure  iron  but  saturated  solid  solution  of  cementite  in  iron, 
and,  in  fact,  in  the  7  modification,  which  possesses  the  greatest 
solvent  power  for  the  carbide.  This  solvent  power  is  not  unlim- 
*  Metallurgie,  3,  i  (1906);  ibid.,  3,  811  (1906). 


86  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

ited,  since  the  cementite  itself  will  not  dissolve  iron.  This  limit 
of  solubility  is  reached  at  a  carbon  content  of  2  per  cent,  that  is,  a 
carbide  content  of  30  per  cent.  The  eutectic  of  a  melt  rich  in 
carbon  is  composed  of  cementite  crystals  on  the  one  hand  and 
mixed  crystals  with  2  per  cent  carbon  on  the  other. 

As  is  the  rule  for  the  precipitation  of  solid  solutions,  the 
composition  of  the  solid  solution  differs  from  that  of  the  melt  out 
of  which  it  was  precipitated.  The  solid  solutions  are  poorer  in 


J.OUU 

1500 
1400 
1300 
^1200 

0 

dllOO 
&1000 

a 

H    900 
800 
700 
600 
500 

A 

s^ 

^ 

D 
C' 

\ 

\ 

^v 

Melt 

t 

Me 

lt  + 

X 

s 

/ 

N 

Solid 
V 

Soli 

tion 

X 

x 

/ 

^Fer 
M 

% 

Sol 

dSo 

lutio; 

^ 

\ 

r^ 

/ 

/ 

{ 

JoTid 

Solu 

bion 

+ 

Eut< 

sctic 

B+ 

G 

V 

E 

utec 

ic 

B' 

Cen 

lenti 

,eCr 

^stals 

\ 

I 

MO^ 

^4 

R 

K 

Ferri 

te+ 

Cem 

mtitt 

:+E 

itecti 

cs  ( 

Peril 

•e) 

Per 

iter| 

Per  cent  Carbon 
FIG.  42. — Equilibrium  Diagram  of  Iron-cementite  Alloys  According  to  Benedicks. 


carbon  than  the  liquid  mass.  The  non-coincidence  of  the  crys- 
tallization and  melting  is  indicated  by  the  existence  of  two  lines 
for  the  precipitation  of  solid  solutions  which  appear  in  the 
equilibrium  diagram.  (See  Fig.  42.) 

If  there  is  less  than  2  per  cent  carbon  dissolved  in  the  melt 
the  solid  solution  that  precipitates  will  be  rich  in  metal. 

The  study  of  the  structure  of  the  alloy  which  is  precipitated 
out  of  the  liquid  melt  is  facilitated  by  the  fact  that  the  existing 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  87 

state  at  high  temperature  is  fixed  by  quenching  and  below  a  red 
heat,  slight  transitions  take  place.  It  is,  therefore,  possible  to 
study  all  the  conditions  and  changes  in  structure  by  metallo- 
graphic  methods. 

As  an  illustration  of  the  method,  a  number  of  photographs 
of  the  structure  of  chilled  specimens  with  a  content  of  more  than 
2  per  cent  carbon  are  given  here.  The  structure  of  a  eutectic 
pig  iron  is  shown  in  Fig.  43.  By  resolution  through  strong  mag- 


FIG.  43.— Cementite-solid  Solution  Eutectic  (Goerens),  Dark  Particles  solid  Solu- 
tion, Light  Cementite. 

nification  (Fig.  44)  it  can  be  seen  to  consist  of  bright  cementite 
crystals  and  a  dark  component,  the  finely  divided  solid  solutions. 

If  we  increase  the  carbon  content  long,  well-formed  cementite 
needles  occur,  which  are  embedded  in  the  eutectic  (Fig.  45), 
strongly  magnified  (Fig.  46) . 

The  primary  precipitated  solid  solutions  in  the  under- 
eutectic  alloys  show  similar  figures  to  ammonium  chloride  pre- 
cipitated from  water,  namely,  pine-tree  formed  crystal  skeletons 
which  are  unmistakable.  They  appear  in  the  picture  (Fig.  47) 


88  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


FIG.  44.— Cementite-solid  Solution  Eutectic  Greatly  Enlarged  (Goerens),  Dark 
Particles  Solid  Solution,  Light  Cementite.     X  750. 


FIG.  45. — Cementite  Needles  in  Cementite-solid  Solution  Eutectic. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  89 


FIG.  46.— Cementite  Needles  in  Eutectic,  Greatly  Enlarged  (Goerens).     X7SQ. 


FIG.  47 —Dark  Solid  Solution  Embedded  in  Cementite-solid  Solution  Eutectic 

(Goerens).     Xio. 


90  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

dark  and  stratified  in  a  bright  eutectic.  The  details  can  be 
seen  in  the  strong  enlargement  (Fig.  48). 

The  unsaturated  iron-cementite  solid  solutions,  as  are 
observed  in  chilled  steel  of  less  than  2  per  cent  carbon,  do  not 
differentiate  themselves  so  sharply  as  the  previously  mentioned 
structural  components  (Fig.  49). 

The  dilute  solid  solutions,  which  are  the  principal  component 
of  a  hardened  steel  are  designated  as  martensite.  It  consists  of  a 


FIG.  48.— Dark  Solid  Solution,  Greatly  Enlarged  (Goerens).     X75o. 

solid  solution  of  cementite  in  iron,  and  this  solution  exactly  as 
in  the  case  of  liquids,  is  capable  of  separating  into  its  com- 
ponent parts.  This  segregating  phenomena  is  also  expressed  in 
the  equilibrium  diagram  (Fig.  42). 

All  steels  show  a  point  of  arrest  in  their  cooling  curves  at 
710°  C.  This  temperature  marks  the  lower  limit  for  the  stable 
existence  of  the  solid  solution;  below  this  temperature  it  breaks 
up  entirely  into  a  mixture  of  iron  and  cementite  crystals.  This 
is,  therefore,  the  eutectic  temperature  of  the  solid  solutions;  it 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


91 


FIG.  49.— Martensite  (Goerens). 


FIG.  50.— Wrought  Iron  (a-Ferrite)  with  Embedded  Slag  Particles  (Goerens). 

Xso. 


92  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

contains  0.85  per  cent  carbon  corresponding  to  12.75  Per 
carbide. 

The  two  solubility  curves  intersect  at  the  eutectic  point, 
exactly  as  is  the  case  with  liquid  solutions.  One  of  these  curves 
represents  cementite  and  the  other  iron.  In  both  cases  the  solu- 
bility decreases  with  falling  temperature.  By  gradual  cooling 
of  suitable  concentrations,  a  precipitation  of  a  component  takes 
place  in  the  solid  state  before  the  eutectic  temperature  is  reached, 


FIG.  51. — Wrought  Iron,  greatly  Enlarged  (Slag  Particles  Dark).     (Goerens). 

X5oo. 

what  that  is  depends  naturally  on  whether  the  carbide  content 
of  the  solution  is  greater  or  less  than  the  eutectic.  Miscro- 
scopically  the  over-eutectic  steel  consists  of  cementite  em- 
bedded in  the  eutectic,  the  under-eutectic  steel  of  iron  crystals, 
and,  indeed,  iron  crystals  in  the  magnetic  form.  This  structure 
component  is  designated  as  ferrite. 

Its  appearance  is  very  characteristic.     It  occurs  as  the  prin- 
cipal constituent  in  wrought  iron  poor  in  carbon  and  can  be  recog- 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  93 

nized  by  the  polygonal  form  of  its  crystals.     They  are  shown  in 
Figs.  50,  51  and  52,  under  different  magnifications. 

Perlite  and  its  Transition  Products. 

The  cementite  ferrite  eutectic  has  the  name  perlite.  Fig.  53 
shows  this  structural  component  with  strong  magnification.  The 
two  components  are  deposited  over  each  other  in  lamellae  and 
form  a  fine  net  work.  The  light  reflection  phenomenon  of  the 
lamellae  gives  similar  effects  as  mother  of  pearl.  This  schiller- 
ization  is  noticeable  with  the  naked  eye  and  this  led  Sorby  to 
call  it  the  pearl-like  component. 


FIG.  52.  —  a-Ferrite  (Goerens). 


Fig.  54  shows  a  steel  which  consists  of  lamellar  perlite  in 
which  is  embedded  ferrite,  Fig.  55,  with  embedded  cementite. 

The  fineness  of  the  perlite  lattice  is  dependent  on  the  dura- 
tion of  heating  and  the  speed  of  cooling.  The  interval  between 
the  lamellae  is  greater  the  longer  time  the  piece  is  subjected  to 
high  temperatures.  This  relation  is  entirely  what  would  be 
expected  from  what  we  learned  in  the  first  chapter  of  the  ten- 
dency of  ferrite  crystals  to  grow,  if  they  were  held  for  a  long 
time  at  a  temperature  between  600-700°.  If  we  treat  steel  con- 


94  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


FIG.  53. — Lamellar  Perlite.     X75Q- 


FIG.  54. — Under-eutectic  Steel.     Lamellar  Perlite +Ferrite  (Goerens).     X7SO. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  95 

taining  perlite  in  this  way  the  same  process  goes  on.  Both  the 
fine  ferrite  particles  and  the  cementite  crystals  are  enlarged,  the 
latter  frequently  growing  together.  In  this  way  the  lamellar 
structure  is  lost  and  a  crystalline  structure  replaces  it  as  shown 
in  Fig.  56.  The  gradual  decomposition  of  the  solid  solutions 
and  the  single  phases  of  the  transition  process,  which  finally  leaves 
perlite  as  the  end  product  can  be  conveniently  studied  from  the 
pictures.  The  products  of  partial  disintegration  differ  in  their 
relations  as  regards  etching  agents  and  have  often  been  claimed 
as  special  structure  components  and  the  names  austenite,  sor- 
bite  and  troostite  given  to  the  various  members  of  the  series. 


FIG.  55. — Over-eutectic  Steel.    Lamellar  Perlite + Cementite  (Goerens).     Xsoo. 

It  must  be  remembered,  however,  that  they  are  not  sharply 
defined  or  the  boundaries  between  them  definite.  They  have, 
accordingly,  no  place  in  the  equilibrium  diagram  as  this  repre- 
sents only  equilibrium  relations.  These  members  appear  in  the 
transition  of  attacked  systems  whose  further  change  is  made  im- 
possible by  quenching. 

It  is  well  to  consider  austenite  as  the  solid  solution  saturated 
with  iron-carbide.  If  a  chilled  white  pig  iron  of  3-4  per  cent 
carbon  is  annealed  cautiously  for  a  short  time  it  changes  its 
action  toward  the  so-called  Kurbatoff  reagent  (a  mixture  of  nitric 
acid  and  acetic  anhydride  in  the  ratio  of  4-100  diluted  with  a 
mixture  of  equal  parts  of  methyl,  ethyl  and  amyl  alcohol). 


96 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


While  the  solid  solution  is  not  previously  attacked  by  this, 
part  of  it  shows  a  dark  color  after  thermal  treatment.  The 
colored  component  has  been  called  troostite.  Benedicks  * 
assumes  that  it  consists  of  cementite  and  ferrite  particles  and 
that  these  possess  ultramicroscopic  dimensions  and  place  troos- 
tite on  a  parallel  with  colloidal  solutions. 


FIG.  56.— Granular  Perlite,  Cementite  Globulites  (in  Relief),  in  a  Ground  Mass 
of  Ferrite  (Goerens).     X75<>- 

A  continuation  of  the  heating  causes  an  enlargement  of  the 
structure  and  we  obtain  sorbite  and,  if  the  lamellar  structure  is 
resolvable  with  the  help  of  the  microscope,  perlite.  Figs.  57  and 
58  show  pictures  of  these  two  intermediate  states  between  the 
solid  solution  and  perlite. 

The  steps  of  the  rearrangement  are  shown  in  the  following 
equation: 

Austenitev 

^jTroostite— >Sorbite— ^lamellar  Perlite— »Granular 
Martensite'  Perlite 

*  Z  .Chem.  Ind.  Kolloide,  1910  p.  290. 


ALLOYS  OF  METALS  WITH  CARBIDES,   ETC.  97 


FIG.  57.— Troostite  (Black);    Austemite  (Light),  Shot  Through  with  Cementite 
Needles  (Goerens).     Xioo. 


FIG.  58.— Sorbite,  Over  Light  Ferrite  (Goerens).     Xioo. 


98  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

If  perlite  be  heated  above  the  eutectic  point,  710°,  a  solid 
solution  of  iron  and  carbide,  martensite  forms.  This  structural 
element  may  be  fixed  by  quenching  and  is  recognized  by  its  hard- 
ness, being  the  constituent  of  hardened  steels.  By  heating  and 
slow  cooling  it  separates  again  into  its  components  and  softens. 
Steel  hardened  by  heating  and  quenching  is  accordingly  in  the 
metastable  state. 

The  Precipitation  of  Carbon  in  Iron  Carbon  Alloys. 

Under  certain  conditions,  namely  slow  cooling  of  the  melt  or 
continued  heating  at  high  temperature  the  carbon  does  not 
precipitate  out  of  the  concentrated  melt  as  cementite  but  as  the 
stable  form,  graphite,  or,  at  relatively  low  temperatures  as 
amorphous  carbon;  the  so-called  temper  carbon.  Due  to  its 
small  specific  gravity  it  collects  at  the  surface  of  liquid  pig  iron, 
which  is  held  somewhat  above  its  melting  point,  as  in  casting 
practice,  and  is  called  "  kish."  In  the  slowly  solidified  gray 
pig  iron  it  is  possible  to  recognize  the  graphite  leaves  with  the 
naked  eye. 

The  equilibrium  between  carbide,  iron  and  graphite  and  also 
that  between  carbide,  iron  and  temper  carbon  are  unknown.  It 
is  certain,  however,  that  amorphous  carbon,  for  example,  sugar 
carbon  as  well  as  finely  divided  graphite  dissolves  in  iron  with  the 
formation  of  carbide.  It  is  the  general  conception  that  the  total 
dissolved  carbon  in  the  melt  is  contained  as  carbide;  we  may 
further  suppose  that  an  equilibrium  is  established  between  this 
solution  and  solid  carbon,  which  can  be  formulated. 

Fe+C  (solid)  «=»  Fe3C  (dissolved). 

It  is  even  possible  that  the  equilibrium  relations  would  also  hold 
for  the  solid  iron-carbide  solutions. 

That  this  equilibrium  has  not  previously  been  established  is 
due  to  the  fact  that  the  dissolved  carbide  especially  in  not  very 
concentrated  solutions  is  very  slowly  decomposed.  It  has, 
therefore,  been  impossible  up  till  now  to  establish  the  eutectic 
point  for  the  precipitation  of  a  mixture  of  graphite  and  solid 
solution  by  cooling  experiments.  These  led  always  to  the  eutec- 
tic point  of  the  metastable  system  solid  solution  cementite. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  99 

The  equilibrium  relations  set  forth  above  do  not  hold  for  solid 
crystallized  cementite,  this  is  always  labile  and  its  decomposi- 
tion velocity  in  the  neighborhood  of  the  solidification  point  of 
pig  iron  (1130°  C.)  is  very  considerable.  This  fact  has  led  a  num- 
ber of  authors  to  assume  that  the  precipitation  of  graphite  must 
always  be  preceded  by  a  precipitation  of  cementite  crystals  in 
the  melt  followed  by  the  irreversible  reaction. 

Fe3C  (solid)  -»  Graphite-f-melt. 


FIG.  59. — Graphite  Druse,  Surrounded  by  Light  Eutectic  (Goerens).     Xso. 

With  pig  iron  of  small  carbon  content  this  indirect  way  of  decom- 
position is  favored,  however,  with  melts  very  rich  in  carbon  in 
which,  according  to  the  law  of  mass  action,  there  must  be  a  very 
great  decomposition  velocity  the  possibility  of  the  direct  crys- 
tallization of  graphite  is  not  rejected. 

Since  the  graphite  is  stable  compared  to  the  cementite,  the 
carbon  content  of  the  melt  with  which  the  graphite  is  in  equili- 
brium is  smaller  than  that  of  the  melt  out  of  which  the  solid 
cementite  crystallizes.  These  relations  must  also  hold  for  the 
solid  solutions  which  are  in  equilibrium  with  the  graphite.  This 


100  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

deduction  is  confirmed  by  the  quenching  test  of  graphite-con- 
taining iron-carbon  alloys  which  do  not  contain  foreign  sub- 
stances, especially  silicon.  The  part  of  the  solid  solution  that  is 
directly  in  contact  with  the  graphite  is  always  carbon  poorer 
than  that  at  a  distance  from  which  the  carbon  has  not  sep- 
arated. In  the  microphotograph  this  fact  can  be  at  once  recog- 
nized. In  Fig.  59  is  seen  a  light  solid  solution-cementite  eutectic; 
in  the  center  of  this  is  a  dark  field  consisting  of  carbon  poor  iron 


FIG.  60.— Graphite  in  Eutectic,  Strongly  Enlarged  (Goerens).  ' 

inside  of  which  are  easily  recognized  graphite  grains.  These 
relations  are  shown  still  better  and  under  stronger  magnification 
in  Fig.  60. 

The  necessary  experimental  material  is  not  extant  for  the 
setting  up  of  a  complete  equilibrium  diagram  of  iron-graphite.  A 
schematic  representation  must  suffice.  From  our  earlier  consid- 
erations it  can  be  deduced  that  the  curves  which  express  the  solu- 
bility of  graphite  in  the  melt  and  in  the  solid  solution  are  de- 
flected to  the  left  as  compared  to  the  cementite  curve.  Also  an 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC 


101 


analogous  equilibrium  line  which  lies  in  the  space  between  the 
graphite  and  cementite  curves  is  to  be  given  for  the  amorphous 
carbon.  (Fig.  61.) 

In  practice,  cases  are  not  met  in  which  the  graphite  pre- 
cipitation goes  on  till  the  equilibrium  is  established.  Micro- 
scopic investigation  of  all  pig  iron  shows  greater  or  smaller 
portions  with  the  structure  constituents  of  the  metastable 


Solid  Solution -F   ' 
Eutectic 


Eutectic  + 
Cementite  Crystals 


Perlite+  Cementite 

Per  cent  Carbon 


6  Cementite 
Fe3C 


FIG.  61. 


systems.     The  necessary  time  for  equilibrium  is  even  greater 
than  the  duration  of  cooling  of  the  preparation. 

If  the  gray  pig  iron  is  heated  for  a  long  time  at  1100°  and 
quenched  in  ice  water  there  are  obtained,  as  would  be  expected, 
graphite  and  the  needle-like  martensite  as  the  structural  com- 
ponents; unchanged  cementite  crystals  are  also  often  met  in 
such  specimens.  In  single  cases  troostite  and  austentite  occur. 
If  the  cooling  is  allowed  to  proceed  slowly  the  specimen  shows 
graphite  and  cementite  embedded  in  lamellar  perlite.  These 


102  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


FIG.  62. — Graphite +Martensite  (Goerens). 


FIG.  63.— Gray  Pig  Iron;  Black  Particles  of  Graphite,  Light  Cementite  Em- 
bedded in  Lamellar  Perlite.  A  Needle  Scratch  is  on  the  Surface,  which 
does  not  Scratch  the  Hard  Cementite.  (Goerens).  Xsoo. 


ALLOYS  OF  METALS  WTIH  CARBIDES,  ETC.  103 

are  the  components  that  one  finds  in  normal  gray  pig  iron.  A 
micrograph  of  such  a  specimen  is  shown  in  Fig.  63 ;  to  indicate 
the  hardness  of  the  different  components  a  needle  scratch  is 
drawn  over  the  surface,  showing  in  the  soft  perlite  as  a  deep 
furrow  while  the  hard  cementite  is  entirely  uninjured.  A  pic- 
ture of  gray  pig  iron  with  strong  graphite  precipitation  is  shown 
in  Fig.  64. 

By  long-continued  heating  of  white  pig  iron,  the  carbon  is 
not  obtained  as  crystal  leaves,  but  in  the  finely  divided  state,  as 
the  so-called  temper  carbon,  which  is  probably  amorphous. 


FIG.  64.—  Gray  Pig  Iron  ;  Black  Particles  of  Graphite,  Light  of  Cementite.     Ground 
Mass  Lamellar   Perlite  (Goerens). 


This  process  has  been  followed  metallographically  by  Wiist. 
The  pictures  of  his  preparations  are  given  in  Figs.  65,  66  and  67. 
Fig.  65  shows  the  original  material,  a  white  pig  iron  which 
has  been  rather  quickly  cooled.  It  consists  as  can  be  seen  of  an 
intimate  mixture  of  cementite  and  solid  solution  which  already 
shows  a  tendency  to  decomposition;  it  is  "sorbitic."  By  heat- 
ing 50  hours  in  vacuum  at  980°  and  slow  cooling  the  structure 
is  totally  changed.  This  can  be  recognized  from  Figs.  66  and  67. 
One  obtains  black  excretions  which  are  surrounded  with  light 
halos,  embedded  in  lamellar  perlite.  The  excretions  are  temper 
carbon  which  have  surrounded  themselves  with  a  circuit  of  fer- 
rite.  It  would  be  interesting  to  see  -how  a  specimen  which  had 


104  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

been  heated  for  a  long  time  and  then  quenched  would  be  con- 
stituted, it  would  then  be  shown  whether  the  carbon  excretions 
result  equally  well  from  isothermal  decomposition  of  cementite 
into  carbon  and  solid  solution,  poor  in  carbon,  or  whether  these 
flocks  are  a  secondary  condition  and  a  result  of  the  precipitation 
and  the  crystallization  of  ferrite  as  we  have  seen  in  the  formation 
of  granular  perlite.  The  practical  use  of  this  process  in  the 
malleabilizing  of  cast  iron  is  known  to  all.  At  temperatures 


FIG.  65.— White  Pig  Iron;  Cementite  (Light) +Sorbite  (Dark)  (Goerens).     Xsoo. 

below  700°  the  decomposition  of  cementite  into  its  elements  is 
still  unobservable  and  would  require  a  very  long  time. 

The  Use  of  Additions  to  Iron  and  Steel. 

The  various  kinds  of  iron  and  steel  which  are  met  in  practice 
are  seldom  pure  iron  carbon  alloys.  Generally  they  contain 
still  other  elements.  We  shall  not  consider  here  such  undesirable 
elements  as  sulfur  and  phosphorus  which  are  excluded  as  far  as 
possible,  but  only  those  which  are  intentionally  added.  Of  these 
manganese  and  silicon  are  especially  important  and  for  the 
preparation  of  special  steels  nickel,  chromium,  tungsten,  molybde- 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


105 


FIG.  66.— Material  of  Fig.  65  Heated  to  900°  and  Cooled  Slowly.  Black  Nuclei 
of  Temper  Carbon,  Surrounded  by  Light  Ferrite;  Ground  Mass  Perlite 
(Goerens). 


FIG.  67.— Material  of  Fig.  66  with  Great  Magnification  (Goerens). 


106 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


num  and  vanadium.  They  all  modify  the  properties  of  the  pure 
iron  carbon  alloys. 

We  will  not  develop  here  the  space  diagrams  of  these  ternary 
and  quaternary  alloys  but  will  confine  ourselves  to  a  general  dis- 
cussion. 

The  changes  which  these  substances  produce  in  the  properties 
of  iron-carbon  alloys  can  be  different  in  nature.  They  depend 
on  the  fact  that  either  the  stability  of  the  carbide  is  lessened  or 
increased  or  that  the  transition  temperature  and  the  eutectic 
point  and  with  them  the  boundaries  of  the  solid  solution  fields  are 
displaced.  It  has  been  known  for  a  long  time  that  the  presence 
of  silicon  in  iron  favored  the  decomposition  of  the  carbide  into 
its  elements;  it  has  even  been  supposed  that  the  presence  of  sil- 
icon was  a  necessary  preliminary  condition  for  the  occurrence 
of  graphite;  this,  however,  has  been  disproved  by  the  investi- 
gations of  Wiist  on  pure  iron-carbon  alloys  which,  by  sufficiently 
long  heating  near  the  melting  point  leave  graphite.  In  all  the 
cases  considered  the  presence  of  this  substance  in  iron  decreases 
the  stability  of  cementite  and  the  solubility  of  the  carbide  in 
liquid  iron.  Investigations  of  the  lowering  of  the  solubility  of 
carbon  in  iron  by  the  addition  of  silicon  have  been  carried  out  by 
Petersen.*  In  carbon  saturated  silicon  containing  alloys,  which 
have  been  heated  for  a  long  time  at  the  melting  point  the  con- 
tent of  carbide  (cementite+ dissolved  carbide)  decreases  with 
rising  temperature  as  the  following  table  shows: 


Silicon, 
Per  Cent. 

Carbon, 
Per  Cent. 

F.  P. 

Silicon, 
Per  Cent. 

Carbon, 
Per  Cent. 

F.  P. 

0 

4-3 

1130 

3-25 

3-41 

1187 

0.13 

4.29 

H38 

3-69 

3-32 

"97 

O.2I 

4-23 

H3I 

3-  96 

3-24 

1205 

0.41 

4.11 

1152 

4.86 

3-08 

I2IO 

0.66 

4-05 

"55 

5.06 

2.86 

1215 

1.14 

3.96 

1160 

13-54 

1.94 

1233 

2.07 

3-79 

1185 

26.93 

0.87 

1255 

2.68 

3.56 

1185 

Metallurgie,  3,  8n  (1906). 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  107 

In  contrast  to  silicon  stands  manganese  which  makes  the 
labile  carbide  stable.  This  action  is  easily  understood  when  we 
think  that  the  manganese  cementite  MnsC,  which  corresponds  to 
iron  cementite  FesC  is  a  stable  compound  which  is  not  split  into 
its  elements  and  that  both  cementites  are  isomorphous  and  are 
miscible  in  all  proportions  in  the  solid  state.  As  a  general  rule, 
the  properties  of  isomorphous  mixtures  are  made  up  additively 
from  the  properties  of  their  components  and  hence  the  small 
tendency  of  manganese  carbide  to  decompose  into  its  elements 
is  shared  by  the  iron-carbon  alloys  containing  manganese. 

We  meet  silicon  as  well  as  manganese  in  pig  irons.  The 
white  pig  iron  always  contains  greater  or  less  amounts  of  man- 
ganese while  the  gray  in  which  the  black  graphite  leaves  are  easily 
recognized  on  a  broken  surface  shows  a  content  of  silicon. 

These  two  elements  are  also  not  without  importance  in  the 
iron-carbon  alloys  poor  in  carbon.  The  silicon  occurs  in  the  iron, 
not  in  the  elementary  form  but  in  the  combined  state.  Guertler 
and  Tammann,  who  have  established  the  equilibrium  diagram  for 
the  binary  system  iron  silicon,  found  the  silicide  FeSi  and  con- 
sider the  existence  of  a  second  silicide  Fe2Si  probable.  The 
latter  forms  with  a  ferrite  an  unbroken  series  of  solid  solutions, 
promotes  the  occurrence  of  this  structure  component  and  in- 
hibits the  polymorphic  change  as  well  as  the  formation  of  car- 
bide solid  solutions.  There  is  also  a  displacement  of  the  eutectic 
point  toward  the  side  of  the  higher  temperature  so  that  the  solid 
solution  field  of  silicon  containing  steel  is  smaller  than  that  of  pure 
carbon  steel. 

With  manganese  we  have  the  complete  miscibility  of  the  metal 
with  iron  on  the  one  hand  and  the  complete  miscibility  of  the 
corresponding  cementite  on  the  other.  The  presence  of  man- 
ganese causes  a  lowering  of  the  eutectic  point  that  goes  hand  in 
hand  with  a  widening  of  the  solid  solution  field. 

As  for  the  rest  of  the  substances  which  are  added  to  the  iron- 
carbon  alloys  in  the  manufacture  of  special  steels,  nickel,  chro- 
mium, tungsten  and  molybdenum,  their  influence  on  the  equilib- 
rium diagram,  at  least  so  long  as  only  small  percentages  of  the 
third  component  are  considered,  is  entirely  similar  to  that  of 


108  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

silicon  and  manganese.  At  higher  concentrations  new  structure 
components  appear.  Nickel  which  with  a  iron  and  with  7  iron 
forms  solid  solutions  in  all  proportions,  enlarges  the  martensite 
field  and  not  only  lowers  the  eutectic  point  but  also  the  transi- 
tion point  of  the  iron  modifications.  The  higher  of  the  two 
points  is  lowered  in  a  larger  measure  than  the  lower  so  that 
finally  a  direct  transition  of  7  solid  solution  into  a  solid  solu- 
tion takes  place. 

Chromium  and  tungsten  which  favor  the  formation  of  f errite, 
form  with  it  solid  solutions.  The  possibility  of  the  occurrence  of 
martensite  is  not  completely  destroyed  but  the  perlite  eutectic 
point  is  raised  and  the  stable  7  solid  solution  field  lessened. 

The  entrance  of  these  different  metals  into  the  solid  solution 
acts  similarly  to  an  increased  content  of  dissolved  carbide,  that  is, 
the  addition  increases  the  hardness.  We  chance  here  again  on 
the  previously  mentioned  fact  that  solid  solutions  are  harder 
than  their  components.  These  special  steels,  especially  nickel 
steel  have  the  great  advantage  over  the  pure  carbon  steels  that 
in  spite  of  their  hardness  they  are  not  brittle. 

The  strong  "  hysteresis  "  of  the  transition  phenomena  which 
is  characteristic  of  all  ternary  and  quaternary  steels  is  of  practical 
and  theoretical  interest.  This  phenomenon  is  not  entirely  missing 
in  the  binary  system  iron-carbon  but  it  is  not  so  strongly  marked 
as  in  the  special  steels.  In  the  determination  of  the  transition 
point  and  the  eutectic  point  by  the  survey  of  the  cooling  and  heat- 
ing curves  it  is  frequently  observed  that  the  transition  phenomena 
do  not  take  place  at  the  same  point  with  falling  temperature  as 
with  rising  temperature  (Fig.  6).  The  alloy  can  accordingly 
be  under-cooled  or  overheated  before  the  transition  is  released ;  a 
retardation  or  hysteresis  takes  place  which,  in  general,  is  greater, 
the  greater  the  amount  of  the  third  component.  This  phe- 
nomenon is  especially  marked  with  nickel  steel  where  the  transi- 
tion is  from  7  solid  solution  into  a  solid  solution.  The  occur- 
rence of  this  change  is  especially  easy  to  observe  as  the  a  solid 
solution  is  magnetic  and  the  7  solid  solution  is  non-magnetic. 
The  change  can  be  recognized  by  the  appearance  or  the  disap- 
pearance of  the  magnetism. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


109 


The  position  of  the  magnetic  transition  point  on  cooling  and 
heating  and  its  relation  to  the  nickel  content  is  given  in  the  fol- 
lowing table  which  is  taken  from  the  work  of  Osmond  *  who  has 
done  the  most  toward  the  explanation  of  these  phenomena. 


COMPOSITION  OF  THE  NICKEL  STEEL. 

MAGNETIC  TRANSITION 
POINT. 

C. 

Ni. 

Si. 

Mn. 

On  Cooling. 

On  Heating. 

0.19   . 

0.27 

0.31 

0.79 

715 

735 

0.19 

3-82 

O.2O 

0.65 

628 

710 

0.17 

7.65 

0.21 

0.68 

530 

710 

0.23 

I5-48 

o.  24 

o-93 

145 

623 

o.  16 

24-75 

o.  24 

0.32 

27 

540 

0.61 

26.  20 

0.24 

0.46 

O 

540 

The  position  of  the  point  is  determined  not  only  by  the  com- 
position, but  also  by  the  thermal  treatment,  which  the  material 
has  undergone.  Osmond's  experiments  on  chrome  steel  show 
that  the  point  of  arrest  on  cooling  varies  considerably  with  the 
temperature  to  which  it  has  been  heated.  The  higher  the  pre- 
vious heating  the  lower  is  the  transition  point  of  solid  solution 
into  perlite. 


Heating  Temperature, 
Degrees  C. 

Transition  Point  on 
Cooling,  Degrees  C. 

835 

713-716 

1030 

682-692 

I22O 

635-643 

1320 

640-600 

These  hysteresis  phenomena  of  steel  are  not  without  analogy 
in  other  fields.  It  is  a  known  and  often  observed  fact  that  a 
labile  modification  of  a  polymorphic  substance  which  is  scarcely 
obtainable  with  the  pure  substance  may  be  relatively  stable  if 
the  substance  is  impure  or  if  a  foreign  substance  is  intentionally 
added.  Heating  to  a  high  temperature  acts  in  a  like  sense. 
It  has  been  deduced  from  these  facts  that  at  high  temperatures, 

*  Compt.  rend.,  128,  304  (1898). 


110  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

the  crystal  neuclei  which  start  the  occurrence  of  the  stable 
crystals  at  low  temperatures  are  destroyed  and  the  more  com- 
pletely the  higher  the  temperature. 

The  solubility  of  the  solid  solution  is  especially  great  in 
quaternary  steels,  in  which  two  metals  are  added  simultaneously 
to  the  carbon-iron  alloy.  In  the  chromium  tungsten  steels,  espe- 
cially if  they  be  exposed  before  cooling  to  a  very  high  tempera- 
ture it  is  not  necessary  to  quench  in  order  to  obtain  them  hard 
since  there  is  practically  no  change  to  the  stable  perlite  form, 
the  precipitation  of  cementite  out  of  the  solid  mass  being  extraor- 
dinarily hindered.  This  steel  also  does  not  lose  its  hardness  by 
heating  to  600°  as  does  ordinary  steel.  In  order  to  obtain  the 
perlite  structure  in  a  chrome-tungsten  steel  it  is  necessary  to  heat 
it  for  a  long  time  due  to  its  very  slow  transformation  which 
requires  at  least  one  hour  at  a  temperature  of  700°.  This  kind 
of  steel  is  used  as  a  tool  steel  since  it  possesses  the  great  advan- 
tage that  it  may  become  hot  during  use  without  losing  its  hard- 
ness. The  quaternary  steels  which  remain  hard  on  heating  are 
called  "  high-speed  steels. " 

There  are  other  metal,  metal-compound  systems  which  are  of 
importance  in  metallurgy  and  we  will  study  here  a  few  of  the 
systems. 

Alloys  of  Metals  and  Oxides.    Copper — Copper  oxide. 

Holborn  and  Day  f  always  observed  lower  values  for  the 
solidification  point  of  copper  in  the  presence  of  air  than  in  an 
indifferent  gas  atmosphere.  The  air  must,  accordingly,  form  with 
the  liquid  metal  a  soluble  product  which  causes  a  depression 
of  the  copper  melting  point.  This  product  can  be  no  other 
than  cuprous  oxide. 

Concerning  the  solubility  of  this  substance  in  liquid  copper 
and  the  solidification  phenomena  as  well  as  the  structure  of  the 
solidified  melt  Heyn  *  has  made  a  thorough  study  and  has 
arrived  at  the  important  result  that  the  alloys  of  metals  and  sub- 
oxides  differ  in  no  respect  from  the  alloys  of  metal  pairs.  The 

*E.  Heyn,  Mittelungen,  aus  den  Konigl.  Techn.  Versuchsanstalt,,  18,  320 
(1900). 

t  Ann.  Phys.  4,  99  (1901). 


ALLOYS  OF  METALS  WITH   CARBIDES,  ETC. 


Ill 


mutual  solubility  of  the  two  components  exists  only  in  the  liquid 
state,  a  solid  conglomerate  of  the  two  components  exists  in  the 
solid  state.  The  typical  solidification  diagram  is  obtained  with 
two  limbs  which  cut  at  the  eutectic  point  (see  Fig.  68).  The  de- 


1180 
1160° 
1140 

1120° 

1100° 
1080° 


Cu-Cu20 


0 


5         6 
FIG.  68. 


8          9         10$Cu20 


pendence  of  the  solidification  point  on  the  oxide  content  is  shown 
in  the  following  table:     (The  temperatures  are  probably  high). 


Content  of 
Cu2O,  Per  Cent. 

SOLIDIFICATION. 

Begins, 
Degrees  C. 

Ends, 
Degrees  C. 

0.08 

IIO2 

1.16 

IOQS 

1085 

i-75 

1089 

1084 

3-5 

1084 

1084 

3-4 

1084 

1084 

4-7 

1116 

1084 

6-3 

1149 

1084 

9.0 

1186 

1084 

The  structure  of  the  crystallized  copper  is  shown  in  the 
accompanying  pictures  (Figs.  69-71).  Fig.  69  shows  the  typical 
eutectic  structure  (compare  to  this  Fig.  20) .  Fig.  70  is  a  picture 
of  an  alloy  which  is  poorer  in  oxygen;  Fig.  71,  one  which  is  richer 


112 


THE   PHYSICAL  CHEMISTRY  OF  THE  METALS 


in  oxygen  than  the  eutectic.  In  the  first  are  shown  particles  of 
the  eutectic  embedded  between  large  flakes  of  metal,  in  the 
latter,  grainy  deposits  of  oxide  in  the  eutectic.  From  the  amount 


FIG.  69.— Cu-Cu20  Eutectic  (Heyn). 


FIG/;©.— Cu-Cu2O  Eutectic  Scattered  Through  Metal  (Heyn). 

of  these  grains  the  oxygen  content  can  be  approximately  ascer- 
tained. The  metallographic  method  which  is  quite  simple  to 
use  can,  in  this  case,  displace  the  analytical.  This  method  is  of 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


113 


practical  importance  since  conclusions  in  regard  to  the  mechanical 
properties  and  usability  of  the  copper  metal  can  be  drawn  from 
the  oxygen  content.  A  high  oxide  content  lowers  the  mallea- 
bility of  the  material  very  essentially. 

Silver — Silver-oxide. 

Liquid  silver  also  dissolves  its  oxide.  It  at  least  takes 
oxygen  from  the  air  which  is  given  up  again  on  solidifying. 
Since  the  oxide  cannot  exist  in  the  free  state  at  the  melting  point 


FIG.  71-— Granules  of  Cu20  in  Cu2O-Cu  Eutectic  (Heyn). 

of  silver  it  breaks  down  into  its  elements  as  soon  as  the  concen- 
tration in  the  melt  is  large,  thereby  showing  the  phenomenon  of 
sprouting.  The  content  of  dissolved  oxide  is  likewise  recog- 
nized, in  that  the  melting  point  of  silver  determined  in  the 
absence  of  air  is  higher  than  that  ascertained  in  the  presence  of 
air,  the  temperatures  are  961.5°  and  955°  respectively.  (Hoi- 
born  and  Day.) 

There  are  also  cases  in  which  the  solubility  of  the  oxide  in 
the  metal  occurs  in  the  solid  state,  for  example,  palladium  oxide 
forms  a  solid  solution  with  palladium  metal. 


114 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


Alloys  of  Metals  and  Sulfides. 

The  number  of  alloys  of  sulfides  with  metals  is  much  greater 
than  that  of  oxides  with  metals.  It  also  happens  that  a  large 
number  of  sulfides  show  metallic  properties  and,  accordingly, 
these  alloys  are  in  many  respects  similar  to  those  of  elementary 
metals.  Partial  and  complete  miscibility  in  the  solid  and  liquid 
state  and  formation  of  compounds  are  known  in  these  alloys  as 
in  those  of  metals. 

These  systems  are  also  of  practical  importance.  They  play, 
for  example,  a  role  in  the  so-called  precipitation  process,  that  is, 
the  smelting  process  by  which  the  metals  are  precipitated  out  of 
their  sulfides  by  decomposition  with  metallic  iron. 

Lead— Lead  Sulfide. 

We  meet  in  case  of  lead  a  slight  solubility  of  the  sulfide  in 
the  metal.  The  solidification  relations  of  this  solution  have  been 
investigated  by  K.  Friedrich  and  A  Leroux.*  The  crystalliza- 
tion diagram  would  be  entirely  normal  if  the  solubility  of  the 
sulfide  in  the  neighborhood  of  the  melting  point  of  lead  were  not 
so  very  small.  The  result  is  that  the  eutectic  point  is  very  close 
to  the  melting  point  of  lead.  The  dependence  of  the  solidifica- 
tion point  on  the  composition  of  the  melt  is  given  in  the  following 
table: 


COMPOSITION. 

F.  P. 

Eutectic 
Point. 

COMPOSITION. 

F.  P. 

Eutectic 
Point. 

PbS. 

Pb. 

PbS. 

Pb. 

100 

94.6 

1103 
1085 

43-0 
38-0 

57-0 
62.0 

1036 
1030 

328 
329 

5-4 

327 

89-3 

10.7 

1073 

327 

33-2 

66.8 

1033 

329 

83-9 

16.1 

1060 

327 

28.3 

71.7 

1015 

331 

78-7 

21.3 

1054 

326 

23-5 

76-5 

998 

325 

73-5 

26.5 

1057 

326 

18.7 

8i-3 

956 

325 

68.3 

31-7 

1050 

325 

14.0 

86.0 

931 

326 

63-2 

36-8 

1047 

329 

9-3 

90.7 

881 

325 

58.i 
53-o 
48.0 

41.9 

47.0 
52.0 

1041 
1049 
1040 

329 
327 
327 

4-7 

95-4 

IOO 

806 

326 
327 

*  Metallurgie,  2,  536  (1905). 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


115 


From  this  and  from  microphotographs  it  appears  that  both 
components,  whether  forming  solid  solutions  with  one  another  or 
compounds,  go  into  sulfides  somewhat  poorer  in  sulfur.  Fig.  72 


FIG.  72.— Pb-PbS  Alloy,  78.7  Per  Cent  PbS;  PbS  Light,  Metal  Dark  (Friedrich 

and  Leroux). 

shows  the  metallograph  of  a  sulfide  rich  (78.7  per  cent),  Fig.  73 
that  of  a  sulfide  poor  (18.7  per  cent)  alloy.  The  same  structure 
components  can  be  recognized  in  both. 


FIG.  73— Pb-PbS  Alloy,  18.7  Per  Cent  PbS;  PbS  Light,  Metal  Dark  (Friedrich 

and  Leroux). 

Antimony — Antimony  Sulfide. 

In  the  system  antimony — antimony  sulfide — we  meet  partial 
miscibility  in  the  molten  state.     These  relations  have  been 


116 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


somewhat  thoroughly  investigated  by  the  French  investigators 
Grunchant  and  Chretien  *  on  the  one  hand  and  by  Pelabon 
on  the  other.  They  show  that  the  molten  sulfide  dissolves  small 
amounts  of  the  metal  and  that  thereby  the  melting  point  of  the 
solvent  is  depressed.  From  the  magnitude  of  this  depression 
the  molecular  weight  of  the  metal  can  be  ascertained;  it  is  as  in 
most  other  cases  monatomic,  the  observed  value  being  113  as 
compared  to  the  theoretical  120.  The  depression  constant  has 
the  value  790. 

At  larger  concentrations  a  separation  occurs  into  two  layers 
of  different  density,which  can  be  observed  up  to  the  boiling  point 
of  the  metal. 

DENSITY  OF  MIXTURES  OF  Sb  AND  Sb2S3 


Temperature  (in  degrees)  .  . 
Sb  layer 

13 

6  7<; 

643 

698 
6  « 

1116 

1156 

6    4."\ 

Density  Sb2Ss  layer 

6  4.3 

*  8<? 

^  82 

The  composition  of  the  layers  of  metal  is  given  in  the  fol- 
lowing table: 


Temp., 
Degrees. 

Per  Cent. 
Sb. 

Temp.. 
Degrees. 

Per  Cent 
Sb. 

Temp., 
Degrees. 

Per  Cent 
Sb. 

Temp., 
Degrees. 

Per  Cent 
Sb. 

539 

11.28 

698 

16-5 

825 

2O.  O 

1130 

21-3 

595 

13.2 

702 

16.0 

960 

20.  6 

1167 

21.  2 

640 

14-34 

750 

17.96 

1036 

21.  0 

1180 

21.  I 

660 

I5-72 

800 

20.  i 

1108 

21.8 

Copper — Copper  Sulfide. 

The  relation  in  the  liquid  state  for  alloys  of  copper  with 
copper  sulfide  is  similar  and  has  been  thoroughly  investigated  by 
E.  Heyn  and  O.  Bauer.f  We  have  here  also  two  melts  differ- 
ing in  their  density,  a  metal  melt  containing  sulfide  and  a  sul- 
fide melt  containing  metal.  At  1102°  the  composition  of  the 
two  layers  is  Cu2S  :  Cu-8  :  81  and  85  :  15,  respectively.  There 
exists  accordingly  a  miscibility  gap.  The  composition  has  un- 
fortunately not  been  studied  at  other  temperatures.  At  the 
*  Compt.  rend.,  142,  709  (1906).  f  Metallurgie,  3,  73  (1906). 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


117 


above-named  temperature  the  two  melts  begin  simultaneously 
to  precipitate  copper  sulfide.  On  cooling  somewhat,  the  layer 
rich  in  sulfur,  disappears  and  there  is  obtained  only  a  melt  rich 
in  copper,  saturated  with  sulfide  whose  content  is  dependent  on 
the  temperature.  The  accompanying  crystallization  curve 
arrives  finally  at  the  eutectic  point  where  it  meets  the  precipita- 


c° 

1130 
1120 
1110 
1100 

1090 

1085. 
1080 

1070 
D 

1060 

B 
H 

SOLII 

OF 

MFICATI 
THE  ALL 

)N  DIAG 
OY  SER 

RAM 
ES 

'1 

COPPI 

IR-COP 

3ER  SUL 

FIDE 

/ 

lOo- 

11 

12 

13 

15 

16 

,    "^ 

/ 

-1102  C° 

9? 

'A  / 

2       / 

VJ 

oJ&iL>0 

Eute 

:tic  Line 

1067  C° 

oE 

°c 

)0           90            80            70            60            50            40            30            20            10             0 
r        10           20          30          40           50          60           70           80          '90          100$Cu2S 

FIG.  74. 

tion  curve  for  the  metallic  copper.  Besides  these  curves  there  is 
between  1102°  and  the  melting  point  of  the  sulfide  1127°  still  a 
further  curve  which  represents  the  precipitation  of  sulfide  from 
melts  rich  in  sulfur  (line  BG  in  Fig.  74).  Fig.  74  shows  the  equil- 
ibrium diagram.  The  eutectic  point  lies  at  1067°.  The  com- 
position of  the  eutectic  alloy  is  3.8  per  cent  sulfide  and  96.2  per 
cent  metal. 


118  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  microscopic  structure  of  the  solidified  melt  is  again  very 
characteristic  and  it  is  possible  exactly  as  with  the  presence  of 
oxide  to  ascertain  very  small  amounts  of  sulfur  in  copper  in  the 
microscopic  way.  Heyn  and  Bauer  found  amounts  that  were 
overlooked  by  the  analytical  methods,  viz.,  o.oi  per  cent  sulfide 
corresponding  to  0.002  per  cent  sulfur  as  strings  of  sulfide  eutectic. 
A  series  of  section  pictures  which  represent  different  parts  of 
the  equilibrium  diagram  are  given  in  Figs.  75-78. 

Fig.  75  shows  the  eutectic  with  3.8  per  cent  sulfide  content 
(magnification  117).  Fig.  76  represents  an  alloy  with  0.49  per 


FIG.  75.—  Cu-Cu2S  Alloy,  Eutectic  3.8  Per  Cent  Cu2S  (Heyn  and  Bauer).     Xn?. 

cent  sulfide  (magnification  117.)  Fig.  77  represents  an  alloy 
with  8  per  cent  sulfide,  the  same  shows  noticeable  sulfide  crystals 
(magnification  117.)  Fig.  78  shows  an  alloy  of  95  per  cent 
with  metal  inclusions  (magnification  117). 


Silver—  Silver  Sulfide. 

Nearly  the  same  equilibrium  diagram  as  described  above  is 
found  for  the  system  silver  —  silver  sulfide,  which  has  been  inves- 
tigated by  Friedrich  and  Leroux  *  the  miscibility  of  the  com- 

*  Metallurgie,  3,  361  (1906). 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  119 


FIG.  76.— Cu-Cu2S;    0.49  Per  Cent  Cu2S;    Strings  of  Eutectic  in  Ground  Mass 
of  Metal  (Heyn  and  Bauer).     Xn?. 


FIG.  77— Cu-Cu2S;    8  Per  Cent  Cu2S;    Noticeable  Sulfide  Crystallites  (Heyn 
and  Bauer).     Xn7. 


120  THE   PHYSICAL   CHEMISTRY  OF  THE  METALS 

ponents  in  the  liquid  state  is  also  limited  here.  There  exists  a 
mixing  interval  (at  906°  for  solutions  with  a  content  between  1.3 
and  97  per  cent  sulfide).  .  For  all  mixtures  whose  composition 
lies  between  these  limits  there  exists  a  constant  temperature  for 
the  precipitation  of  metal  (906°).  Solutions  with  a  content  less 
than  13  per  cent  show  higher  crystallization  temperatures;  the 
corresponding  curve  limb  has  its  origin  in  the  melting  point  of 
the  pure  metal.  The  remaining  curve  limb,  the  limb  for  the  pre- 
cipitation of  the  sulfide  out  of  the  rich  melt  which  starts  at  the 
melting  point  of  that  compound,  835°  and  that  for  the  crystalliza- 


FIG.  78.-^Cu-Cu2S;  Metal  Inclusion  K  (Heyn  and  Bauer).     Xn?. 

tion  of  metal  from  high  percentage  melts,  which  turns  off  hori- 
zontally at  906°,  are  yet  to  be  observed,  only  the  eutectic  tem- 
perature at  which  the  two  cut  is  known,  namely,  807°.  In  the 
cooling  curves  of  the  alloys  there  is  shown  a  still  further  point  of 
arrest  at  175°  at  which -an  allo tropic  transition  of  the  sulfide 
occurs. 

Iron — Iron  Sulfide. 

The  partial  miscibility  between  metals  and  sulfides  appears 
to  be  the  rule;  at  least  we  find  still  others  like  the  systems  con- 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


121 


sidered.  Treitschke  and  Tammann*  found  these  phenomena 
also  for  the  mixtures  of  iron  with  iron  sulfide,  since  the  two  liquids 
are  very  viscous,  layer  formation  does  not  take  place,  but  an 
emulsion  of  the  two  melts  forms.  The  mixing  gap  at  1400°  lies 
between  20  and  92  per  cent  metal.  The  miscibility  of  the  com- 
ponents is  also  continued  in  the  solid  state  but  is  only  partial. 
A  solid  solution  of  little  sulfide  is  known  in  7  iron  (c)  and  also  of 


1600 1 
1500 
1400 

FeSisool 

1200 


p.1 

I  300 1 
200 1 
100 


Solid  Solution 
a  Melt 


1100  H^ 

Solid 

1000 
Solution!  [[) 

900 
Melt 

800 

10 

.700 

°>HI 


Solid 


2  Liquids 


/3  Solid  Solution  D+  So 


l-9 

"^iT 


Solution  D+  OL 


a  Fe  S  +ac.Fe 


Iron 


id  Solution^ 


n 


V 


ron 


1540  Fe 


970 

850 
780 


130 


0   5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  85  90  95  100 

Per  cent  by  Weight  Fe 

FIG.  79. 


iron  in  0  sulfide  (D).  The  first  separate  on  cooling  with  the 
precipitation  of  iron  (along  ik).  Thereby  the  solid  solution  is 
enriched  in  sulfide  up  to  a  content  of  4  per  cent  at  780°,  below 
this  temperature  it  decomposes  to  a  solid  mixture  of  a  ferrite  and 
solid  solution  D  which  at  128°  undergo  an  allotropic  modi- 
fication into  a  sulfide.  The  complicated  equilibrium  diagram 
is  shown  in  Fig.  79. 


Z.  anorg.  Chem.,  49,  320  (1906). 


122 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


Nickel— Nickel  Sulfide. 

The  diagram  for  alloys  of  nickel  and  nickel  sulfide  studied 
by  Bornemann*  which  is  given  in  Fig.  80  is  still  more  complicated. 
The  only  sulfide  capable  of  existence  in  the  molten  state  has  the 

Temp. 
1500  hNi 


1400 


1300 


1200 


1100 


1000 


FIG.  80. 


composition  Ni3S2;  at  low  temperatures  still  further  compounds 
exist  namely  NiS,  NisS^  NiS2  and  apparently  also  NieSs.  As  in 
all  nickel  containing  systems,  the  tendency  to  form  solid  solu- 

*  Metallurgie,  5,  13  (1908). 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


123 


tions  is  very  great;  there  precipitates  from  the  melt  principally 
solid  solutions  which  separate  from  the  melt  in  a  very  complex 
way. 

Alloys  Between  Sulfides. 

In  the  metallurgy  of  copper,  nickel  and  lead,  there  occur  as 
important  intermediate  products,  the  so-called  mattes,  copper 
matte,  lead  matte  and  nickel  matte,  alloys  of  the  corresponding 
sulfides  with  iron  sulfide.  It  is  of  importance  to  obtain  a  picture 
of  their  important  structural  constituents.  In  many  cases 
compounds  occur  between  the  components,  in  part  these  are  such 
as  occur  in  nature  as  ores,  we  have  accordingly  before  us  a  field 
which  must  arouse  the  interest  of  the  smelterman  to  a  high 
degree.  Here  also  metallographic  methods  give  the  explana- 
tion of  the  nature  and  formation  of  these  products. 

In  most  cases  the  equilibrium  diagrams  are  comparatively 
simple.  Out  of  the  melts,  miscible  in  all  proportions  the  com- 
ponents crystallize  on  solidification  in  the  pure  state.  The 
diagram  consists  of  two  curves  which  cut  in  a  eutectic  point. 
This  form  we  find  in  all  alloys  in  which  lead  and  zinc  sulfide 
occur  as  components.  The  melting  points  of  the  components, 
the  position  of  the  eutectic  temperature  as  well  as  the  composi- 
tion of  the  eutectic  alloy  are  given  in  the  following  table: 


COMPONENTS. 

MELTING  POINT. 

Eutectic 
Tempera- 
ture. 
Degrees  C. 

Composition 
of  Eutectic. 

Observer. 

A. 

*  ! 

A. 
Degrees  C. 

B. 
Degrees  C. 

PbS 

FeS 

1114 

1187 

863 

70%  A 

PbS 

AgS     | 

III4 

835 

630 

77%  A 

PbS 

CuS    i 

III4 

II2I 

535 

49%  A 

• 

ZnS 
ZnS 

PbS 

AgS     : 

1600 
1600 

1114 
835 

1044 
807 

6%  A 

3%  A 

K.  Friedrich* 

ZnS 

FeS 

1600 

1171 

1162 

5%  A 

ZnS 

CuS 

1600 

II2I 

{    near    } 

I     H2I     / 

? 

• 

*  Metallurgie,  4,  479  (1907);  4,  672  (1907);  5.  114  (1908). 

There  are  also  cases  in  which  solid  solutions  occur  between 
the  components;    in  the  system  copper  sulfide,  silver  sulfide 


124 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


which  has  been  investigated  by  Friedrich  *  there  appears  to  be 
present  complete  miscibility  in  the  solid  state.  The  solidifica- 
tion curve  has  a  minimum  at  677°.  The  following  table  shows 
the  dependence  of  the  solidification  point  on  the  composition: 


COMPOSITION. 

COMPOSITION. 

AgjS. 

Cu»S. 

AgjS. 

CuzS. 

p.p.    0 

IOO 

835 

40 

60 

701 

90 

10 

749 

30 

70 

876 

80 

20 

698 

20 

80 

945 

70 

30 

677 

/           IO 

90 

1050 

60 

40 

688 

IOO 

1  121 

50 

50 

719 

According  to  the  investigation  of  P.  Rontgen  there  are  a 
number  of  compounds  which  occur  between  the  components  of 
copper  matte;  those  recognizable  by  a  maximum  in  the  solidi- 
fication diagram  are  (Cu2S)s.FeS,  Cu2S-FeS  and  probably  a 
third  (Cu2S)2(FeS)5;  also  in  the  system  silver  sulfide — antimony 
sulfide  we  find  a  maximum  at  the  places  on  the  diagram  which 
represent  the  composition  of  the  compounds  Sb2S3Ag2S  and 
Sb2Sa-3Ag2S.  Both  occur  naturally,  the  first  as  miargyrite,  and 
the  last  as  pyrargyrite  or  ruby  silver  ore. 

The  position  of  the  melting  point  of  the  components  and  com- 
pounds (maxima)  as  well  as  the  co-ordinates  of  the  eutectic 
point  are  given  for  the  two  systems  in  the  following  table : 


Components  and 
Compounds. 

Melting 
Points  and 
Maxima., 
Degrees. 

Eutectic 
Temp., 
Degrees. 

Composition  of  Eutectic 
Alloy. 

Observer. 

Cu2S  

io8< 

QQC 

Cu2S,  21.8%;  FeS,  78.2% 

P.  Rontgen 

(Cu  S)3FeS  .  . 

106$ 

Cu2S  FeS  
(CuiSMFeS),.... 
FeS  
Sb2S3  

1030 
980 

1133 
562 

1000 

895 

Cu2S,  67%;  FeS,  33% 
Cu2S,  33;  FeS,  67% 

Sb283,Ag2S  
Sb2S3  •  3  Ag2S 

503 

438 

44-O 

Sb2S3,82.S%,Ag28,i7.5% 
SboSs  46  *%•  AffoS  <C2  7% 

Pelabon 

Aft.S 

8?o 

AC:  A 

Sb2S3   2i%-  Ag2  8  79% 

*  Metallurgie,  4,  671  (1907). 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


125 


We  meet  very  complex  relations  in  the  nickel  sulfide — iron  sul- 
fide  alloys.  The  equilibrium  diagram  is  given  by  Bornemann 
(Fig.  81).  The  sulfides  FeS  and  Ni3S2  combine  with  each  other 
to  a  compound  (FeS)  2  NisS2  which  is  capable  of  forming  solid 
solutions  with  the  nickel  sulfide  in  all  proportions;  at  low  tem- 
peratures different  transformations  go  on,  the  nature  of  which  is 
not  entirely  clear.  Also  the  nickel  sulfide  Ni2S  not  existent  in 

W.S. 


1200  z 


1100 


1000 


500 


FIG.  81. 

the  pure  state  forms  with  the  iron  sulfide  a  complex  compound. 
Out  of  the  melt  crystallizes  (FeS) 2  Ni2S  which,  on  cooling  to 
575°,  splits  up  into  (FeS) 3  (Ni2S)2  and  iron  sulfide.  At  still 
lower  temperatures  there  forms  from  the  double  sulfide  with 
excess  of  iron  sulfide,  the  further  compound  (FeS)  4  Ni2S. 

Phosphorus  and  Arsenic  Containing  Alloys. 

Phosphides  may  also  dissolve  in  a  molten  metal  and  are 
capable  of  forming  solid  solutions  to  a  certain  degree  in  the  solid 
state. 


FeS 


126  THE   PHYSICAL   CHEMISTRY  OF  THE  METALS 

There  is  at  least  the  case  of  copper  phosphide  CusP  which  is 
soluble  up  to  1.2  per  cent  in  solid  copper  and  also  the  case  of  iron 
phosphide. 

The  system  copper — phosphorus  has  been  investigated  by 
Heyn  and  Bauer.*  Copper  phosphide  CuaP  and  phosphide 
saturated  copper  crystals  are  the  occurring  structure  components. 
The  melting  point  of  the  pure  phosphide  lies  at  1023°.  The 
eutectic  alloy  of  these  components  melts  at  708°  and  contains 
8.27  per  cent  P.  Alloys  very  rich  in  phosphorus  which  contain 
more  than  14.1  per  cent  (representing  CuaP)  are  made  up  of 
a  solid  solution  of  the  above-named  phosphide  with  a  second 
which  has  the  possible  composition  CusP2. 

In  the  system  iron — phosphorus,  J.  E.  Stead  f  ascertained  the 
occurrence  of  compounds  of  composition  FeaP  and  Fe2P.  The 
alloys  of  the  metal  and  the  phosphide  FeaP  show  in  their  equi- 
librium diagram  great  similarity  with  those  previously  men- 
tioned. Solid  solutions  of  iron  with  phosphide  which,  as  a 
maximum,  contain  1.7  per  cent  P,  that  is,  10.9  per  cent  FeaP 
also  form  here. 

Alloys  rich  in  phosphide  consist  of  a  eutectic  of  10.2  per 
cent  phosphorus,  in  which,  with  a 'content  of  under  10.2  per  cent, 
solid  solutions  are  embedded;  with  10.2-15.6  per  cent  well- 
formed  rhombic  phosphide  crystals  are  embedded.  The  eu- 
tectic itself  is  a  fine  mixture  of  saturated  solid  solution  with 
phosphide  crystals.  The  pure  phosphide  FesP  contains  15.6 
per  cent  phosphorus  and  melts  at  1060°.  Under  the  micro- 
scope the  single  structure  constituents  can  be  readily  recognized. 
On  this  basis  we  give  several  microphotographs  of  the  same 
here  (see  Figs.  82-85). 

Alloys  with  still  higher  phosphorous  contents  consist  of  the 
two  phosphides  FeaP  and  Fe2P,  which  can  be  easily  separated  in 
the  powdered  state  by  means  of  a  magnet.  The  compound  poor 
in  phosphorus  is  magnetic,  while  the  other  is  not.  Concerning 
the  equilibrium  diagram  in  this  field  no  close  investigation  has 
been  made. 

*  Metallurgie,  4,  242,  287  (1907). 

t  J.  Iron  and  Steel  Inst,  1900,  II,  60. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC. 


127 


The  smelterman  meets  arsenic  containing  alloys*  in  the 
working  of  arsenical  ores.  By  reduction  of  previously  roasted 
ores  one  obtains  the  so-called  speiss;  alloys  of  arsenides  of  dif- 


FIG.  82 .— Fe-Fe3P  Alloy;   10.2  Per  Cent  P;  Eutectic  (J.  E.  Stead). 


FIG.  83.— Solid  Solution  of  Fe  with  Fe3P.     (J.  E.  Stead).     Xso. 

ferent  metals.  The  nickel  and  cobalt  speiss  that  occurs  as  an 
intermediate  product  in  the  reduction  of  nickel  from  arsenical 
nickel  ores  is  a  mixture  which  consists  essentially  of  iron  and 

*  For  arsenic  alloys  in  general,  see  K.  Freidrich,  Metallurgie,  3,  41  (1906); 
4,  200  (1907). 


128  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

nickel  arsenides.  To  get  a  glimpse  into  the  constitution  of  this 
speiss,  K.  Friedrich  and  his  co-workers  carried  out  a  series  of 
investigations  on  the  alloys  of  metals  with  arsenic.  They  have, 


FIG.  84.— Fe-Fe3P  Alloy;  1.8  Per  Cent  P;  Three  Polygonal  Solid  Solution  Crystals, 
the  Interstices  are  Filled  with  Eutectic.     (J.  E.  Stead).     X350. 


FIG.  85.— Fe-Fe3P  Alloy;    11.97  Per  Cent  P;    Crystals  of  Fe3P  Embedded  in 
Eutectic  (J.  E.  Stead).     X6o. 

so  far  as  possible,  established  the  crystallization  diagram  and 
investigated  the  solidified  mixtures  metallographically  for  their 
structure  constituents,  the  diagrams  can  not  be  completed  as 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  129 

arsenic  is  a  volatile  element  and  is  distilled  by  high  temperatures 
from  quite  dilute  melts.  The  nature  of  the  measurements  allow 
the  investigation  of  only  those  solutions  whose  vapor  pressure 
is  lower  than  atmospheric. 

The  relations  of  the  lead  arsenic  alloys  are  very  simple,  both 
components  separating  in  the  pure  state.  The  pictures  show 
distinct  arsenic  crystals  but  no  compounds.  The  eutectic  tem- 
perature is  292°  and  the  melt  contains  2.5-3.0  per  cent  arsenic. 
The  eutectic  point  of  the  silver  alloy,  527°,  has  been  established 
by  the  cooling  experiments.  Compounds  as  constituents  do 
not  have  to  be  considered  here  and  the  same  is  true  of  the  system 
zinc — arsenic. 

Two  compounds  are  formed  by  melting  together  arsenic 
and  copper.  The  melting  point,  that  is,  the  solidification  max- 
imum of  one,  CusAs  lays  at  830°,  the  maximum  for  the  second 
CusAs2  is  hidden,  since  the  latter  compound  changes  at  710°, 
into  the  first.  The  eutectic  data  for  the  mixture  of  copper  with 
CusAs  is  683°  and  an  arsenic  content  of  22  per  cent,  for  the  mix- 
ture of  CusAs2  with  an  unknown  arsenic-rich  product  is  600° 
and  47  per  cent.  In  the  existence  field  of  the  arsenide  CusAs2  a 
transition  point  of  307°  is  observed;  what  change  takes  place 
there  is  not  known. 

The  speiss-formers  proper,  iron  arsenide  and  nickel  arsenide, 
have  been  carefully  investigated  and  with  them,  is  known  with 
certainty,  the  occurrence  of  compounds  on  the  one  hand  and  of 
solid  solutions  on  the  other.  The  constitution  of  these  alloys 
can  best  be  recognized  from  the  equilibrium  diagram.  (Figs. 
86  and  87.) 

With  iron  arsenic  (see  Fig.  86)  five  different  kinds  of  crystals 
are  observed  inside  of  the  interval  91.6-44  per  cent  iron.  Under 
this  we  find  with  certainty  the  two  arsenides  Fe2As2  and  Fe2As 
possibly  also  FeAs.  The  solidification  point  of  the  last  two  lay 
at  919°  and  1030°,  respectively.  The  compound  FeaAs2  does 
not  crystallize  directly  from  the  melt  but  is  the  product  of  a 
transformation  which  takes  place  at  800°  and  with  an  arsenic 
content  of  40  to  57  per  cent.  The  nature  of  the  iron  rich  crystals 
which  precipitate  out  of  the  melt  at  iron-contents  between  92 


130 


THE   PHYSICAL   CHEMISTRY   OF  THE   METALS 


1500 

1400° 

1300° 

1200C 

1100 

1000 

900 

800C 

?o6c 

400C 


0 
100 


FeAs 


Field  of  existence  of 

FesAs3 


10 


15 

.85 


20 
'80 


35 

'65 


FIG.  86. 


50 
<50 


45 


«     N1 


1500 


Ni  As 


FIG.  87. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  131 

and  70  per  cent  is  not  entirely  clear,  but  they  are  probably  a 
solid  solution  of  iron  and  some  of  its  arsenides.  By  cautious 
cooling  there  occurs  at  1000°  a  transition  in  which  crystals  of 
FeAs  react  with  the  melt  with  the  formation  of  a  kind  of  crystals 
of  which  one  conceives  either  as  a  further  compound  Fe5As4 
or  as  a  solid  solution.  With  quick  cooling  this  reaction  does  not 
always  take  place. 

With  the  nickel  arsenic  alloys  (Compare  Fig.  87)  there  is 
shown  the  probable  existence  of  the  compounds  NisAs2  and 
NiAs,  also  possibly  NisAs2.  The  solidification  maximum  for 
Ni5As2  is  found  at  998°,  that  for  NiAs  at  968°.  The  first  of  these 
compounds  experiences  an  allotropic  transformation  that  takes 
place  not  far  below  the  melting  point.  As  with  all  nickel  com- 
pounds we  meet  here  also  the  decided  tendency  for  the  forma- 
tion of  solid  solutions.  We  observe  limited  miscibility  between 
NisAs2  and  metallic  nickel  with  a  mixing  interval  between  5.4 
and  33.5  per  cent  arsenic  and  also  a  slight  solid  solution  forma- 
tion between  the  two  arsenides  NisAs2  and  NiAs;  in  these  the 
first  of  the  two  components  is  the  solvent.  The  arsenic  richer 
crystallizes  out  of  the  melt  in  the  pure  state.  The  compound 
NisAs2  does  not  form  directly  out  of  the  liquid  melt  but  owes  its 
existence  to  a  decomposition  in  the  solid  state  which  takes  place 
in  the  temperature  interval  700-750  degrees. 

Silicides  of  the  Metals. 

Concerning  the  constitution  of  the  metallurgically  important 
silicide  systems  the  investigations  of  Tammann*  and  of  Ru- 
dolphi  |  have  been  submitted. 

Iron  silicon  compounds  that  we  have  already  met  in  the  dis- 
cussion of  special  steels,  are  of  the  greatest  importance;  we  men- 
tioned there  that  an  iron  silicide  with  a  crystallization  maximum 
of  1143°  is  known  and  that  the  existence  of  a  second  Fe2Siis 
probable  which  forms  with  iron  a  continuous  series  of  solid 
solutions.  The  crystallization  of  the  pure  compound  out  of  the 
melt  can  only  take  place  inside  of  a  very  limited  field  of  concen- 
tration, and  accordingly,  in  the  microscopical  way  is  difficult  to 
*  Z.  anorg.  Chem.,  49,  93  (1906).  f  Z.  anorg.  Chem.,  53,  216  (1907). 


132 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


recognize.  It  occurs,  however,  in  a  eutectic  in  which  the  second 
component  is  the  silicide  FeSi.  The  formation  of  this  takes 
place  at  1242°  and  34.6  atom-per  cent  of  silicon.  The  eutectic 
point  for  a  mixture  of  silicon  and  the  compound  FeSi  lies  at  a 
concentration  of  75  atom-per  cent  and  a  temperature  of  1240°. 
The  equilibrium  diagram  (Fig.  88)  shows  best  the  relation  of  the 
occurring  structure  components  to  each  other. 

Out  of  the  liquid  melt  of  copper  and  silicon  there  precipi- 
tates, from  suitable  concentrations,  the  compound  CuaSi.     The 


104U 

1500 
1400 
1300 
1200 
1100 

1 

1425 

\ 

vr 

N\\ 

L 

c 

\ 

I 

\ 

\ 

/ 

/ 

\ 

\   / 

/ 

f 

\ 

A 

d 

\ 

I 

d 

b 

e 

f 

h 

i 

k 

)   10   20   30   40   50   60   70   80   90   100 
FIG.  88. 

melting  point  of  the  same  lies  at  862°  (crystallization  maximum). 
It  forms  with  the  metal  two  series  of  solid  solutions  (a  and  b) 
which  merge  into  each  other  with  the  melt  at  849°.  A  eutectic 
of  the  solid  solution  b  and  CuaSi  is  met  at  820°  and  the  content 
of  the  melt  of  9.79  per  cent  silicon.  The  eutectic  consisting  of 
Si  and  CusSi  crystals  forms  at  811°  and  an  Si  content  of  the 
melt  of  17.61  per  cent.  In  the  cooling  of  the  solidified  mass  re- 
actions take  place.  Between  780°  and  815°  the  solid  solution 
b  breaks  up  into  solid  solution  a  and  a  third  kind  of  solid  solu- 
tion c,  which  in  turn  goes  over  at  710°  into  a  solid  solution, 
and  crystals  of  the  composition  CuigSi*. 


ALLOYS  OF  METALS  \\TIH  CARBIDES,  ETC.  133 

Still  more  complex — as  always  in  the  presence  of  nickel- 
are  the  relations  of  the  system  nickel-silicon.  We  meet  there, 
according  to  the  investigations  of  Guertler  and  Tammann* 
the  compounds  Ni2Si,  NiaSi2,  NiSi  and  probably  also  N^Sis, 
of  these  the  first  forms  a  solid  solution  series  with  the  metal  with 
a  mixing  interval.  It  is  also  possible  to  form  such  a  series  with 
NiSi.  The  latter  compound  N^Sis  dissociates  into  silicon  and 
melt  so  that  its  crystallization  maximum  is  concealed,  on  the 
other  hand  it  occurs  in  an  a  and  0  modification.  The  silicide 
NisSi2  results  through  a  change  in  the  solid  state.  It  would 
be  going  too  far  to  enumerate  all  the  peculiarities  here. 

The  Phase  Rule. 

We  will  now  turn  to  a  general  question,  after  we  have  learned 
the  large  number  of  single  systems  and  the  great  multiplicity  of 
phenomena  which  we  can  have  with  alloys  of  metals  with  other 
metals,  and  of  metals  with  compounds  of  metallic  character,  or, 
finally,  with  alloys  of  such  among  themselves. 

We  have  seen  that  with  our  material  all  different  kinds  of 
structural  components  may  appear  side  by  side.  We  have  seen 
the  components  as  such  in  the  solid  state,  we  see  them  in  com- 
pounds with  one  another,  we  have  learned  their  capability  of 
forming  solid  solutions  with  one  another  or  with  the  compounds. 
The  elements  as  well  as  the  compounds  are  frequently  capable  of 
existing  in  many  modifications.  A  great  abundance  of  phe- 
nomena, concerning  the  liquid  melt  and  the  vapor,  we  have  not 
once  mentioned. 

We  are  driven  to  the  question:  "  Can  all  the  different  forms 
we  have  met  in  the  complex  alloy  systems — say  in  the  alloys  of 
nickel  and  sulfur  exist  simultaneously  side  by  side  or  is  there  a 
definite  limit.  The  experience  which  we  have  had  in  these  con- 
siderations has  taught  us  that  these  are  only  able  to  exist  under 
determined  external  conditions  of  temperature,  composition 
and  pressure.  These  are  the  factors  which  restrict  the  existence, 
the  appearance  and  disappearance  of  all  possible  forms  of 
matter." 

*  Z.  anorg.  Chem.,  49,  92  (1906). 


134  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

Before  we  proceed  to  the  answering  of  the  question  we  will 
introduce  a  conception  with  which  we  will  operate  much  in  the 
future. 

As  soon  as  we  allow  the  melt  to  solidify  the  product  ceases  to 
be  homogenous;  we  may,  as  the  pictures  show,  recognize  more 
or  less  definitely  different  space  divisions.  These  spaces  of  the 
system,  distinguished  by  chemical  relations  and  by  the  mixture 
relations  of  the  components  are  designated  by  W.  Gibbs  as 
phases.  He  considered,  however,  separate  parts  of  identical 
nature  as  belonging  to  the  same  phase.  At  this  place  it  should 
be  remarked  that  a  solid  eutectic  which  in  metallography  is  con- 
sidered a  single  structural  component  must  be  regarded  as  two 
phases,  it  consists  as  one  may  readily  be  convinced  by  strong 
magnification  of  an  intimate  mixture  of  two  different  kinds  of 
crystals. 

As  phases  we  can  take  the  different  kinds  of  solid  components, 
however,  besides  these  the  liquid  and  vapor  are  to  be  considered 
as  such.  In  a  heterogeneous  system  one  can  usually  have  a 
number  of  phases  together;  vapor  and  liquid,  solid  crystals  and 
melt,  as  we  have  seen  are  often  compatible  with  each  other. 

In  1874,  before  the  development  of  metallography,  the 
American  physicist,  Willard  Gibbs,  derived  the  rule  from  gen- 
eral thermodynamics  which  states,  without  consideration  of  the 
special  chemical  nature  of  the  heterogenous  system,  how  many 
phases,  as  a  maximum  for  a  given  system,  can  exist  side  by  side 
and  be  in  equilibrium  with  each  other.*  The  number  of  phases 
depends  on  the  number  of  variables  present.  From  this  mass  of 
variables  we  designate  the  simplest  chemical  constituents  as  the 
components  of  the  system;  the  different  kinds  of  physical  influ- 
ences to  which  the  system  is  exposed,  also  come,  into  considera- 
tion as  variables.!  The  latter  are  rather  numerous,  we  can 
electrify  and  magnetize  the  system,  we  can  light  it  with  different 
colored  light,  we  can  heat  it  and  treat  it  in  different  ways  mechan- 
ically. If  we,  however,  limit  the  consideration  to  the  action  of 

*  Thermodynamic  Studies,  Trans.  Conn.  Acad.  in  (1874). 
t  For  detailed  applications  of  the  phase  rule,  see  "The  Phase  Rule,"  W.  D, 
Bancroft;  "The  Phase  Rule  and  its  Applications,"  Alexander  Findlay. 


ALLOYS  OF  METALS  WITH  CARBIDES,  ETC.  135 

temperature  and  pressure  the  phase  rule  of  Gibbs  can  be  stated 
as  follows: 

The  sum  of  the  phases  P  and  the  degrees  of  freedom  F,  which 
the  system  possesses  are  equal  to  the  number  of  components  N 
increased  by  2,  the  number  of  physical  variables  temperature 
and  pressure. 

F+P=N+2* 

We  will  now  try,  by  the  use  of  known  examples,  to  make  clear 
the  use  of  the  phase  rule. 

First,  we  consider  a  system  of  one  component,  the  co-existence 
of  the  different  states  of  a  single  substance  which  does  not  suffer 
chemical  change;  we  know  that  vapor  and  liquid  can  exist  side 
by  side  and  the  phase  rule  says  that,  since  the  number  of  phases, 
is  2,  and  the  component  number  is  i,  the  system  must  possess 
one  degree  of  freedom,  that  is,  we  have  freely  at  our  disposal  one 
variable,  temperature  or  pressure  without  thereby  endangering 
the  coexistence  of  the  two  phases.  If  we  have,  at  our  disposal, 
the  temperature,  the  pressure  is  established  at  which  both  states 
can  coexist.  To  every  temperature  there  corresponds  an  en- 
tirely determined  equilibrium  pressure,  the  vapor  pressure.  If 
we  also  have  at  our  disposal  the  pressure  one  of  the  two  phases 
disappears,  by  raising  above  the  vapor  pressure,  the  vapor,  by 
lowering,  the  liquid.  The  two  are  only  stable  side  by  side  if  the 
exact  vapor  pressure  prevails.  If  we  do  not  hold  the  tempera- 
ture constant  and  have,  at  our  disposal,  the  pressure  so  there 
appears  for  every  pressure  an  entirely  determined  boiling  tem- 
perature. Vapor  pressure  and  temperature  are  inseparably  con- 

*  A  more  complete  statement  of  the  phase  rule  is  given  by  T.  W.  Richards, 
J.  Am.  Chem.  Soc.,  37,  464  (1915).  It  is  as  follows:  F—(ni-\-nE)  —  («0-|-wr), 
where  F= variance,  n\  =  number  of  individuals,  HE= number  of  physical  mani- 
festations of  energy  brought  into  play,  «<£  =  the  number  of  phases,  and  wr=the 
number  of  independent  restrictions  placed  on  the  system,  but  not  included  in 
the  definition  of  individuals.  The  individuals  of  any  reacting  system  are  the 
separate  chemical  substances,  undecomposed  in  the  reactions  concerned,  which 
are  necessary  to  construct  the  system.  The  number  of  such  individuals  to  be 
chosen  is  the  smallest  number  necessary  to  construct  the  system. 

It  is  frequently  necessary  in  metallurgical  systems  to  use  this  complete  state- 
ment, otherwise  serious  errors  may  result. 


136  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

nected.  By  the  position  of  one  magnitude,  the  other  is  clearly 
determined.  The  relations  are  precisely  the  same  if  we  consider 
the  equilibrium  conditions  between  a  solid  and  vapor. 

If  we  place  the  demand  for  the  simultaneous  existence  of  three 
phases  the  liquid,  the  solid  and  the  vapor  so  we  have  according 
to  the  phase  rule  no  degrees  of  freedom,  no  possibility  of  con- 
trolling freely  temperature  and  pressure,  equilibrium,  therefore, 
exists  only  at  a  determined  temperature — the  melting  point — 
and  an  entirely  determined  vapor  pressure.  These  demands  of 
the  phase  rule  correspond  now  in  all  cases  to  our  experience.  If 
we,  for  example,  would  have  two  polymorphic  substances  as 
solid  phases  side  by  side  and  in  equilibrium  with  them  vapor  that 
is  only  possible  at  a  determined  transition  point.  If  one  recedes 
from  this  equilibrium  temperature,  a  phase  disappears  over- 
stepping the  melting  point,  causes  melting  of  the  solid,  under 
cooling  solidification  of  the  liquid,  raising  of  the  external  pressure 
causes  condensation  of  the  vapor.  We  come  again  to  a  two- 
phase  system  in  which  every  equilibrium  temperature  corre- 
sponds to  a  determined  equilibrium  pressure.  Also  in  the  system, 
solid  substance — melt  that  is  the  case.  The  pressure  repre- 
sents a  determined  melting  point  and  we  are  able  by  change  of 
the  external  pressure  to  accomplish  a  displacement  of  the  melting 
point.  The  mutual  relations  of  the  two-phase  system  can  be 
seen  graphically  represented  above  (see  Fig.  89).  Every  two- 
phase  system  is  expressed  by  a  curve,  the  three-phase  system  in  a 
point,  the  intersection  point  of  the  three  curves,  melting  and 
transition  points  are  such  triple  points,  starting  points  for  a 
curve  triplet  in  the  temperature  pressure  plane. 

With  two  components  and  two  phases  simultaneously  present, 
one  can  change  either  the  temperature  or  pressure,  without 
the  possibility  of  breaking  up  coexistence.  For  N  =  2  and  P  =  2 
the  phase  rule  gives  F  =  2,  that  is,  two  degrees  of  freedom,  if  we 
consider  the  vapor  and  melt  of  a  mixture  of  two  substances  so 
there  corresponds  to  a  temperature  not  a  single  vapor  pressure 
but  a  multiplicity  of  the  same,  every  displacement  of  the  mixing 
relations  requires  also  a  change  of  vapor  pressure  and  boiling 
temperature. 


ALLOYS  OF  METALS  WITH  CARBIDES,   ETC. 


137 


The  establishment  of  the  vapor  pressure  and  the  composition 
of  the  melt  and  vapor,  results  only  by  the  addition  of  a  third 
phase,  it  is  immaterial  whether  it  be  a  second  liquid  as  with  sub- 
stances of  limited  miscibility  or  whether  it  be  a  solid  component. 

We  recall  from  our  wide  consideration  of  the  case  that  all 
substances  send  off  some  vapor  in  air  or  vacuum,  the  vaporiza- 
tion is  often  very  small  and  escapes  direct  observation.  We 
will,  therefore,  in  all  cases  assume  the  vapor  phase  as  present. 

If  we  have  a  liquid  alloy,  for  example,  of  zinc  and  cadmium, 
its  concentration  as  well  as  the  pressure  and  composition  of  the 
vapor  phase  is  fixed,  if  crystals  of  one  or  the  other  component 


Pressure 


Temperature 


FlG.  89. 


precipitate.  If  we  change  the  temperature  the  quantitative 
relations  change  but  the  possibility  of  coexistence  of  the  phases 
is  not  endangered.  The  freedom  disappears  only  if  both  kinds 
of  crystals  are  precipitated  together  from  the  melt.  These  are 
only  stable  together  at  a  single  temperature,  the  eutectic  tem- 
perature. 

The  degree  of  freedom  for  complex  systems  can  be  deduced 
at  once  from  the  phase  rule. 

There  remains  still  to  mention  a  much-used  and  practical 
nomenclature.  Heterogenous  systems  of  one  degree  of  freedom 
are  designated  as  univariant,  of  two  degrees  of  freedom  as  bivari- 


138  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

ant,  of  three  trivariant,  of  more  than  three  multivariant  The 
system  without  a  degree  of  freedom  is  called  nonvariant. 

What  service  can  the  phase  rule  now  render  us?  It  is  first  of 
use  for  the  systematizing  of  different  combinations,  so  that  we 
can  orient  ourselves,  as  to  the  relations  of  these  with  respect  to 
temperature  and  pressure.  Concerning  the  direction  of  resulting 
phase  displacement  as  well  as  concerning  the  changes  which 
take  place  in  the  inside  of  phases  of  variable  composition-con- 
centration changes  in  solution  and  vapor — it  allows  us  to  make 
no  predictions. 

It  is  frequently  an  important  criterion,  e.g.,  when  it  is  used 
to  establish  whether  a  complex  system  that  possesses  many 
phases  is  in  equilibrium  or  whether  it  is  in  the  labile  state.  An 
excellent  example  of  the  use  of  the  phase  rule  in  this  direction  is 
given  us  in  the  above  treated  system  iron — carbon,  where  we  see 
occurring  as  the  structural  components  of  this  two  component 
system,  besides  the  always  present  gas  phase,  cementite,  graphite, 
solid  solution  or  cementite  graphite  and  ferrite,  over  a  large  tem- 
perature interval.  The  phase  rule  says  at  once  that  theoretically 
this  combination  of  four  phases  can  only  exist  at  one  tempera- 
ture. Accordingly  in  the  interval  of  observation  one  of  the 
present  structure  components  must  be  labile. 

We  see  therefrom  that  in  the  investigation  of  heterogenous 
systems  one  may  in  many  cases  take  advantage  of  the  use  of  the 
phase  rule;  it  is  certainly  suitable  to  facilitate  the  preliminary 
survey.  We  must  remember,  however,  that  it  gives  us  only  the 
numerical  relation  between  the  components  present,  the  phases 
and  the  possible  freedom.  It  does  not  make  possible  a  deeper 
insight  into  the  chemistry  of  the  system. 


CHAPTER  IV 

THE   METALLURGICAL   REACTIONS— OXIDATION   AND 

REDUCTION 

We  have  carefully  considered  in  the  earlier  chapters,  the 
properties  of  metals,  their  changes  of  state,  and  the  phenomena 
which  occur  in  the  mixing  of  metals  with  each  other  and  with 
compounds  of  metallic  character,  and  we  have  seen  the  value  of 
physico-chemical  methods  in  the  field  of  alloys.  The  usefulness 
of  physical  chemistry  in  problems  which  concern  alike  the  chem- 
ist and  the  metallurgist  is  not  thereby  exhausted.  The  reactions 
which  are  used  in  the  laboratory  and  in  practice  for  the  extrac- 
tion of  metals  from  their  compounds  are  elucidated  if  physical 
chemistry  is  applied  to  the  investigation  of  the  equilibrium  con- 
ditions of  the  substances  participating  in  these  reactions. 

The  metallurgical  reactions  give  to  the  physical  chemist  a 
large  number  of  problems  of  very  great  theoretical  as  well  as 
practical  interest,  and  there  is  hardly  a  field  in  which  the  study 
of  chemical  equilibrium  is  so  advantageously  applied.  The 
conditions  which  permit  us  to  make  observations  on  the  equi- 
librium are  here  unusually  simple  since  the  range  of  temperature 
which  the  metallurgist  has  at  his  disposal  is  very  large  and, 
therefore,  within  it  the  equilibrium  is  measurable  for  many 
reactions.  As  the  temperatures  used  are  mostly  high  the 
customary  reaction  velocity  is  usually  sufficiently  great  for  the 
relatively  rapid  attainment  of  the  equilibrium,  accordingly 
changes  of  temperature  or  of  pressure  and  other  conditions  can 
be  conveniently  observed. 

Of  the  reactions  which  have  proved  favorable  for  the  appli- 
cation of  chemical  statics  we  will  treat  the  oxidation  and  reduc- 

139 


140  THE   PHYSICAL   CHEMISTRY  OF  THE   METALS 

tion  processes,  as  well  as  the  decomposition  of  sulfides  with  oxy- 
gen, the  so-called  roasting  processes,  rather  thoroughly. 

Equilibrium  Between  Metal,  Oxide  and  Oxygen. 

We  have  now  come  to  the  consideration  of  the  oxidation  and 
reduction  processes  and  will  try  to  make  clear,  as  the  first  exam- 
ple, the  relations  which  exist  between  the  metals,  their  oxides 
and  gaseous  oxygen. 

It  is  a  well-known  fact  that  a  large  number  of  metals  —  we 
designate  them  as  base  —  are  oxidized  even  at  ordinary  tempera- 
ture, and  more  rapidly  on  heating,  and  it  is  also  known  on  the 
other  hand,  that  there  are  oxides  which  give  off  oxygen  on  heat- 
ing and  leave  the  pure  metal  as  a  residue.  The  experiment  of 
Scheele,  to  break  down  HgO  into  its  elements,  belongs  to  every 
course  of  lectures  in  experimental  inorganic  chemistry.  The 
reactions  between  metals,  oxides  and  oxygen,  can,  accordingly, 
run  in  two  opposite  directions. 

The  question  now  arises,  are  there  not  also  metals,  with  which 
depending  on  the  conditions,  both  directions  of  the  reaction  may 
be  observed?  The  question  is  to  be  answered  "  yes."  Le- 
Chatelier  found,  for  example,  that  silver  is  such  a  metal  and  is 
oxidized  at  a  temperature  of  300°  if  the  oxygen  pressure  is  more 
than  15  atmospheres;  on  the  other  hand  the  oxide  is  decomposed, 
if  the  pressure  is  less  than  10  atmospheres.  The  oxygen  pressure 
accordingly  appears  to  play  an  essential  role. 

The  Application  of  the  Phase  Rule. 

The  kind  of  equilibrium  that  exists  between  oxide,  metal  and 
gaseous  oxygen  can  be  at  once  deduced  from  the  phase  rule.  If 
the  metal  and  oxide  exist  as  separate  phases  we  have  in  all,  with 
the  gas  phase,  three  phases.  The  system  consists  of  two  com- 
ponents, metal  and  oxygen.  The  number  of  degrees  of  freedom  is 
accordingly  (P  =  3  ,  N  =  2). 


The  system  is,  therefore,  univariant.     Every  temperature  corre- 
sponds to  a  definite  oxygen  pressure.     The  process  of  the  oxygen 


THE  METALLURGICAL  REACTIONS 


141 


liberation  from  oxides,  reminds  us  of  the  vaporization  of  a  liquid, 
where  we  also  met  a  definite  gas  pressure,  depending  on  the  tem- 
perature. This  pressure  is  entirely  independent  of  the  relative 
amounts  of  the  two  phases. 

Oxygen  Tensions  of  Oxides. 

Lewis  *  has  found  the  following  values  for  the  decomposition 
tension  of  silver  oxide, 


Temperature  

302° 

325° 

445° 

Pressure  in  atmospheres.  . 

20.5 

32 

307 

Pelabon  f  gives  considerably  smaller  values  for  HgO,  as  fol- 
lows: 


Temperature  
Pressure  in  mm  .  .  . 

440° 
small 

610° 
1240 

The  number  of  metal  oxides  whose  oxygen  tension  is  con- 
veniently observable,  that  is,  somewhere  between  20  and  800 
millimeters,  is  not  large.  A  complete  series  of  observations  on 
palladium  oxide  has  been  made  by  L.  Wohler.J  He  found  the 
values  collected  in  the  following  table : 


Temperature  in 
Degrees. 

Oxygen  Tension 
Solid  Phase 
PdO,  Pd  in 
mm. 

Oxygen  Tension 
Solid  Phase, 
PdO,  Solution  of 
PdO  in  Pd,  in 
mm. 

756 

67 

67 

808 
812 

212 
23O 

239 

840 

414 

483 

850 

510 

566 

864 

634 

766 

In  the  consideration  of  this  table  it  appears  that  the  above 
deduced  relation,  that  every  temperature  corresponds  to  a  def- 

*  Z.  Physik.  Chem.,  55,  449  (1906). 
t  Compt.  rend.,  128,  825  (1899). 
tZ.  Elektrochem.,  12,  781  (1906). 


142  THE  PHYSICAL  CHEMISTRY  OF-  THE  METALS 

inite  pressure,  no  longer  holds.  Wohler  observed  that  the 
equilibrium  pressure  depends  to  a  considerable  extent  on  the 
time  during  which  the  metal  and  oxide  are  heated  together. 
There  occurs  in  this  system  a  special  condition,  that  the  metallic 
palladium  possesses  a  solvent  action  on  the  oxide  even  in  the  solid 
state.  The  metal  phase  will  contain  a  more  or  less  large  amount 
of  this  substance  in  solution  then,  depending  on  the  time  it  is  in 
contact  with  the  oxide  or  oxygen.  If  this  solvent  action  did  not 
exist  every  temperature  would  correspond  to  a  single  oxygen 
pressure,  since,  however,  this  is  not  true,  we  must  be  dealing 
with  a  changing  composition  of  the  metal  phase.  The  tension 
rises  with  rising  oxide  content  and  reaches  its  upper  limit,  in  a 
saturated  solid  solution  of  oxide. 

So  long  as  the  solution  is  not  saturated  the  complete  equilib- 
rium between  the  two  phases,  to  which  alone  the  phase  rule 
refers,  is  still  not  attained. 

If  we  represent  the  results  of  these  measurements  graphically 
we  obtain  for  the  system  with  pure  metal  as  solid  phase,  as  well  as 
for  every  solid  solution  of  given  concentration,  a  curve  whose  form 
is  similar  to  the  form  of  the  vapor  pressure  curve  (see  Fig.  90). 

If  the  external  oxygen  pressure  is  higher  than  the  dissocia- 
tion tension  of  the  oxide,  oxidation  of  the  metal  takes  place, 
while  the  direction  of  the  chemical  process  is  the  opposite,  if  the 
oxygen  pressure  of  the  surroundings  is  less  than  the  tension  of 
the  oxide.  The  curves  which  represent  the  dependence  of  the 
oxygen  tension  on  the  temperature  divide  the  plane  of  the  curve 
into  two  fields  of  reaction.  That  of  higher  pressure  represents 
the  formation,  that  of  lower  pressure  the  decomposition  of  the 
oxide. 

High  decomposition  tension  and  low  decomposition  tempera- 
tures are  found  in  oxides  of  noble  metals,  with  which  we  observe 
ordinarily  only  decomposition.  In  opposition  thereto  the  dis- 
sociation tension  of  the  base  metals  is  considerably  under  the 
oxygen  tension  of  the  atmosphere  (about  150  mm.)  so  that,  only 
the  oxidation  process  is  observed.  The  tension  is  generally  so 
small  that  it  can  no  longer  be  measured  with  the  help  of  our 
ordinary  manometer  methods. 


THE  METALLURGICAL  REACTIONS 


143 


We  are  able  however  to  obtain  the  order  of  magnitude  of  the 
same  indirectly.  Nernst  has  derived  a  formula  by  which  it  is 
possible  to  calculate  the  oxygen  tension  approximately  from  the 
combining  heat  of  the  metals.  The  calculation  for  oxide  of 
iron  has  been  made  by  v.  Juptner,  that  for  the  other  heavy 
metals  by  W.  Stahl.*  The  following  table  gives  us  the  essence 
of  the  work. 

Oxygen  Tension  in  Atmospheres. 


Temper- 
ature, 
Deg.  Abs. 

2AgjO 
4Ag+02. 

2Cu»O 
4Cu  +Oz. 

2PbO 
2Pb  +O«. 

2NiO 
2Ni+O*. 

2ZnO* 
2Zn  +O». 

2FeO 

300 
400 
500 
600 
700 

8.  4Xio-5 
4.9XIO-1 
24.9X10 
360.0 

.56XIO"30 
8.oXio~24 
i.9Xio"19 

3.iXio"38 
9.4Xio~31 

2.IXIO"25 

i.SXio"46 
2.8Xio"31 

i.3Xio~68 
4.6Xio"66 
4.3Xio"47 

S  IXIO"42 

800 
900 
IOOO 



3.6X10-" 
i.7Xio"12 

3.2Xio~18 

8.7Xio"23 
8  4Xio~20 

2.4X10"  40 
4.3X10    35 
7  iXio"31 

9.iXio"30 

2   OXlO"22 

IIOO 

7  8Xio"10 

i  3Xio~13 

2  3Xio~17 

2    oXlO"27 

I2OO 

2    OXlO" 

7  oXio~12 

2  6Xio~" 

i  5Xio~24 

i  6Xio~19 

noo 

3    *XlO~ 

2  iXio"10 

i  4X10"" 

A      7VlO~22 

I4OO 

3.6X10" 

3.8Xio~9 

4  4Xio~12 

«;  4.XIO"20 

S.9Xio~14 

I  'COO 

2  8X10" 

4  8Xio~8 

8  7X10"" 

3  6Xio~18 

1600 

i  8Xio~ 

A    4XlO~7 

I    2XlO~9 

2  8Xio~~n 

1700 

8.9X10" 

3  2Xio"6 

I    2XlO~8 

^  ?Xio~16 

1800 

3  8X10" 

i  8Xio~6 

9  6Xio~8 

6  8Xio~14 

Xi  -' 

1900 

2OOO 



1.4X10" 
4.4X10" 

8.9Xio"6 
3-7Xio-4 

6.iXio"7 
3-3Xio"6 

9.iXio"13 
9-SXio"12 

i.6Xio"7 

*  These  figures  for  zinc  are  purely  hypothetical,  since  at  temperatures  above  the  boiling 
point  of  zinc  we  do  not  have  two  solid  phases,  and  a  gas  phase  giving  a  univariant  system 
but  one  solid  phase  (ZnO)  and  one  gas  phase  (Zn  +O)  which  gives  us  a  bivariant  system. 
The  same  is,  of  course,  true  for  any  volatile  metal. 

That  these  calculations  do  not  give  accurate,  but  only  approx- 
imate values,  is  shown  by  a  comparison  of  the  calculated  pres- 
sure, for  silver  oxide,  with  that  experimentally  determined  by 
Lewis. 

We  may  not,  however,  assume  that  no  oxidation  of  the  metal  is 
possible  at  oxygen  pressures  below  the  tension  of  the  oxide,  only 

*Metallurgie  4,  682,  (1907). 


144 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


the  formation  of  the  pure  oxide  phase  is  impossible.  We  always 
get  oxidation  if  the  metal  has  the  power  to  dissolve  the  oxide, 
with  palladium,  for  example,  of  whose  oxidation  phenomenon 
we  have  already  learned,  a  gradual  absorption  of  the  gas  by  the 
solid  metal  takes  place  at  small  oxygen  pressures.  A  similar 
property  is  shown  by  liquid  silver  which  can  take  up  a  consider- 
able amount  of  oxygen.  This  property  of  silver  is  not  without 
practical  importance.  Before,  however,  we  consider  these 
phenomena  we  will  orient  ourselves,  concerning  the  degrees  of 


700 
600 
500 
400 
300 
200 
100 


02~  Pressure 


700° 


20°   40°   60° 


20° 


40°   60°   80°  900* 


FIG.  90. 


freedom  of  similar  systems,  which  consist  of  only  two  phases, 
the  gas  and  the  metal  solution.  According  to  the  phase  rule 
there  are,  for  a  two-component  system  with  the  simultaneous 
presence  of  two  phases,  two  degrees  of  freedom,  that  is,  we  can 
vary  the  temperature  and  oxygen  pressure  without  one  of  the  two 
phases  disappearing,  an  increase  of  oxygen  pressure,  at  constant 
temperature,  causes  an  increase  in  the  oxide  concentration,  a 
lowering  of  the  pressure,  a  partial  decomposition  of  the  dis- 
solved oxide.  Every  concentration  corresponds  to  a  definite 
oxygen  pressure,  regardless  of  whether  the  solution  is  a 


THE  METALLURGICAL  REACTIONS  145 

solid  or  a  liquid.  A  very  small  amount  of  oxygen  is  theoretically 
sufficient  to  form  a  small  amount  of  dissolved  oxide,  even  if  the 
metal  is  a  noble  one.  There  is,  therefore,  no  paradox,  if  we 
ascribe  the  absorption  of  oxygen  by  metallic  silver,  to  the  forma- 
tion of  oxide,  even  if  the  silver  oxide  in  the  solid  state  cannot 
exist  at  the  same  temperature,  due  to  its  high  dissociation  ten- 
sion. The  amount  of  absorbed  oxygen  is  relatively  small,  always 
forming  very  dilute  solutions.  If  we  cool  below  the  solidification 
point,  the  pure  metal  precipitates  from  the  solution.  The  result 
is  that  the  oxide  in  the  melt,  becomes  more  and  more  concen- 
trated, thereby  raising  the  oxygen  tension  of  the  solution.  By 
quick  cooling  it  rises  above  atmospheric  pressure  and  the  gas 
breaks  with  violence  through  the  crystallizing  mass.  We  say 
silver  "  sprouts." 

A  whole  series  of  metals  combine  with  oxygen  in  a  number 
of  ratios,  for  example,  the  different  oxides  of  iron  FeO,  Fe203, 
Fe304,  those  of  manganese  MnO,  MnaO.*,  Mn203,  of  antimony 
Sb203,  Sb204,  Sb205,  of  lead  PbO  and  Pb02,  of  copper,  Cu20 
and  CuO,  as  well  as  many  more.  For  every  one  of  these  oxides 
there  exists,  if  it  is  in  contact  with  oxygen  and  its  metal,  a  special 
temperature  tension  curve. 

The  Equilibrium  Between  Two  Oxides  and  Oxygen. 

In  the  most  cases,  we  observe,  with  the  escape  of  oxygen 
from  higher  oxides,  not  the  occurrence  of  the  metal,  but  the  form- 
ation, first,  of  the  lower  oxide.  The  relations  between  the  two 
different  degrees  of  oxidation,  existing  as  separate  phases,  and 
oxygen,  are  entirely  analogous  to  those  which  we  studied  in 
the  consideration  of  the  system  oxide  —  metal  —  oxygen.  The 
equilibrium 


corresponds  at  a  given  temperature  to  an  entirely  definite  oxygen 
pressure. 

The  decomposition  of  barium  peroxide  investigated  by  Le- 
Chatelier  *  is  of  practical  importance  and  is  convenient  for 
*  Compt.  rend.,  115,  565  (1892). 


146 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


accessible  observation.     The  equilibrium  is  characterized  by  the 
equation 


The  observed  dissociation  tensions  are  shown  in  the  following 
table  and  Fig.  91. 


Temperature  in 
Degrees. 

Ot-  Tension 
in  Mm. 

Temperature  in 
Degrees. 

Oz-Tension 
in  Mm. 

525 

20 

735 

260 

555 

25 

750 

340 

650 

65 

775 

510 

670 

80 

785 

620 

720 

210 

790 

670 

800 
700 
600 
500 
400 
300 
200 
100 


02  Pressure  m  m. 
2  Ba  02^2Ba  0+02 


2  Pb  Ca  03= 
2  PbCa02+02 


500°     600°     700°     800°     900°    1000°  1100?   1200°  Temp. 
FlG.  91. 

The  reversible  reaction  used  for  the  technical  preparation  of 
oxygen  is  entirely  analogous  to  the  above-mentioned  case.     It  is 

2Ca  Pb03  +±  2Ca  Pb02+O2. 


It  is  treated  here  as  a  complex  oxide.     This,  however,  does 
not  affect  the  results.    Likewise  here,  a  single  temperature  corre- 


THE  METALLURGICAL  REACTIONS 


147 


spends  to  an  entirely  definite  oxygen  pressure,  the  measurements 
for  this  case  have  also  been  made  by  LeChatelier.* 


Temperature 
in  Degrees. 

Oz-  Tension 
in  Mm. 

Temperature 
in  Degrees 

Oz-  Tension 
in  Mm. 

2PbCaO3  

880 

47 

1060 

557 

«=* 

940 

112 

1070 

570 

+± 

950 

117 

IIOO 

940 

2PbCaO2-O2 

IO2O 

3^O 

IIIO 

1040 

The  technical  use  of  this  substance  is  accomplished  by  heat- 
ing it  at  a  temperature,  where  the  tension  is  under  that  of  the 
partial  oxygen  pressure  of  the  atmosphere.  Under  these  con- 


0 2  Pressure  m  m. 


Cu  0  Pure 


960      980°    1000°   1020°   1040°  1060°   1080°  1100° 
FIG.  92. 

ditions  the  air  acts  on  the  lower  oxide  converting  it  into  the 
higher.  .  Then  the  air  is  shut  off  and  the  apparatus  connected 
to  an  air  pump,  which  considerably  lowers  the  pressure  in  the 
reaction  vessel.  Under  the  strongly  reduced  pressure  the  higher 
oxide  is  decomposed  and  the  evolved  oxygen  is  compressed  into 
iron  cylinders.  We  have,  accordingly,  here,  a  direct  use  of  the 
conclusions  which  one  can  deduce  from  chemical  statics. 

If  the  two  oxides  are  soluble  in  one  another,  whether  the  solu- 
tion be  solid  or  liquid,  the  dissociation  pressure  is,  as  in  the  solu- 
tion of  oxide  in  metal,  dependent  not  only  on  the  temperature, 
but  on  the  relative  proportions  of  the  two  oxides.     This  case 
*  Compt.  rend.,  117,  109  (1893). 


148 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


occurs  with  the  oxides  of  copper.  A  mixture  of  cupric  and  cu- 
prous oxides  forms  at  high  temperatures  a  single  phase.  The 
dependence  of  the  oxygen  pressure  of  this  solution  on  the  tem- 
perature and  the  cuprous  oxide  concentration  has  been  studied 
by  L.  Wohler.*  He  found  the  following  values  for  the  reaction: 

2CuO  <=»  Cu2O+O2. 


Temperature 
in  Degrees. 

O2-  Tension. 

Nearly  Pure 
CuO  in  Mm. 

SoCuO,  soCu2O 

in  Mm. 

960 

IOOO 
1010 
IO2O 
1030 
IO4O 
IO5O 
1060 
1070 

5° 
118 
142 

174 
212 
258 
314 
380 
458 

31 
65 

108 

164 

We  are  now  oriented,  concerning  the  different  possibilities 
which  we  may  meet  for  the  systems  consisting  of  oxide,  metal 
and  oxygen.  These  models  form  the  basis  for  the  study  of  the 
reduction  processes,  which  we  will  now  consider. 

Direct  Decomposition  of  Oxides  by  Heat. 

It  is  only  possible  in  a  few  cases,  namely,  with  the  noble 
metals,  gold,  platinum,  mercury,  etc.,  to  obtain  the  metal  from 
the  oxide,  simply  by  the  splitting  off  of  the  oxygen  at  high  tem- 
perature. To  accomplish  this  it  is  necessary  that  the  oxygen 
dissociation  pressure  of  the  concerned  oxide,  be  greater  than  the 
oxygen  pressure  of  the  surrounding  atmosphere.  According 
to  theory  it  is  possible  to  decompose  every  oxide  in  an  absolute 
vacuum.  We  cannot,  however,  obtain  so  complete  a  vacuum 
by  our  present  air  pumps,  as  small  traces  of  gas  remain  in  every 
vacuum  no  matter  how  good  since  the  confining  liquid,  mercury 
or  oil,  has  a  vapor  pressure.  For  the  decomposition  of  nickel  and 
*  Z.  Elektrochem.,  12,  784  (1906). 


THE  METALLURGICAL  REACTIONS  149 

iron  oxide  at  1000°  (to  say  nothing  of  the  oxides  of  less  noble 
metals)  vacuua  are  necessary,  according  to  the  preceding  table 
of  Stahl,  which  possess  a  gas  pressure  of  io~23  atmospheres.  This 
is  far  below  the  limit  of  practical  attainment  and  we  may  dismiss 
at  once  all  experiments  to  decompose  this  kind  of  oxides  direct 
into  their  metals. 

The  so-called  reducing  agents  are  generally  used  to  obtain  a 
metal  from  its  oxide.  The  most  important  of  these  are  the  alkali 
and  alkali  earth  metals,  as  well  as  aluminum  and  manganese, 
further  the  gaseous  reducing  agents,  hydrogen  and  carbon 
monoxide,  and,  first  of  all,  as  technically  the  most  important, 
carbon. 

Reduction  by  Metals. 

The  action  of  the  named  metals  is  at  once  clear.  All  these 
metals  are  base,  that  is,  their  affinity  for  oxygen  is  extraordinarily 
great.  It  is,  therefore,  extremely  tightly  bound  in  the  oxides  and 
the  dissociation  pressure  of  the  same  is  very  small.  If  now,  we 
have  together,  an  oxide  with  high  oxygen  pressure  and  a  strongly 
reducing  metal,  it  can  easily  be  conceived  that  the  latter  would 
demand  the  oxygen  for  itself.  The  equilibrium  of  the  oxide 
with  the  original  metal  and  oxygen,  would  thereby  be  imme- 
diately destroyed  and  the  decomposition  reaction  would  go  on, 
till  all  the  oxygen  had  been  given  to  the  base  metal.  All  the 
metals  can  be  used  as  reducing  agents  for  a  given  oxide  whose 
oxides  possess  a  lower  oxygen  tension  than  the  one  to  be  reduced. 

This  condition  is  fulfilled  in  the  Goldschmidt  process  for 
obtaining  chromium  and  manganese  from  their  oxides  by  means 
of  aluminum,  further,  in  the  iron  industry,  overblown  charges 
are  reduced  with  ferro-manganese. 

Reduction  by  Gaseous  Reducing  Agents. 

The  reduction  of  metallic  oxides  by  gaseous  reducing  agents 
hydrogen  and  carbon  monoxide,  can  only  be  understood  by 
obtaining  a  wider  basis  of  chemical  statics.  We  have  here  a 
favorable  opportunity  to  compare  the  different  methods  of 
working  with  one  another  and  to  learn  their  range  of  use- 
fulness. 


150  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

Reduction  by  Hydrogen. 

The  reduction  of  metallic  oxides  by  hydrogen  is  so  well  known 
that  I  do  not  need  to  name  a  special  example.  If  we  designate 
with  M  any  monovalent  metal,  the  reaction  scheme  is 


In  metallurgy,  on  the  other  hand,  a  process  is  used  by  which, 
with  the  help  of  water  vapor,  easily  oxidizable  metals,  as  zinc,  are 
separated  from  molten  nobler  metals,  as  crude  lead,  which  has 
been  desilverized  with  zinc. 

2M+H2O  <±  M2O+H2. 

It  is  readily  seen  that  this  is  the  reverse  of  the  above  formu- 
lated reduction  reaction.  We  now  look  again  for  a  metal  in 
which  both  reaction  directions  can  be  observed.  It  has  long 
been  known,  that  by  heating  in  a  current  of  steam,  iron  goes 
over  into  FeaCU  and  that,  on  the  other  hand,  this  oxide  is  reduced 
by  hydrogen  to  metal.  This  is  accordingly  a  reversible  reac- 
tion, which  we  write 

Fe304+4H2  +±  3Fe+4H2O. 

We  will  now  study  the  conditions  which  determine  whether 
reduction  or  oxidation  takes  place,  and  will  consider  for  this  pur- 
pose a  system  in  which  all  of  the  substances  entering  into  the 
double  decomposition  may  be  simultaneously  present. 

The  Phase  Rule. 

We  apply  first  the  phase  rule,  with  which  we  are  already 
familiar  and  try  with  its  help  to  come  to  a  conclusion  concerning 
the  nature  of  our  system.  Obviously  it  is  to  be  treated  as  a 
system  of  three  components,  namely,  iron,  hydrogen  and  oxygen 
which,  at  the  reaction  temperature,  are  divided  among  the  three 
phases,  metal,  oxide,  and  gas  (mixture  of  steam  and  hydrogen). 
We  conclude,  therefrom  that  we  have  a  bi  variant  system,  that 
two  degrees  of  freedom  are  present,  and  that  accordingly  we 


THE  METALLURGICAL  REACTIONS  151 

can  vary  the  temperature  and  pressure  without  the  possibility 
of  injuring  the  coexistence  of  the  three  phases. 

The  phase  rule  allows  us  to  say  nothing  concerning  the  com- 
position of  the  gas  phase. 

LeChatelier's  Principle. 

If  the  reaction  is  allowed  to  proceed  in  a  closed  space  and  the 
temperature  held  constant  it  is  impossible  to  observe  the  pro- 
cedure of  the  reaction  by  the  change  of  gas  pressure  since  the 
number  of  water  molecules  resulting  from  the  reaction  is  exactly 
the  same  as  the  number  of  hydrogen  molecules  going  into  it. 
Such  reactions  are  independent  of  external  pressure,  that  is,  a 
pressure  change  exerts  no  influence  on  the  composition  of  the  gas 
phase. 

The  influence  of  pressure  on  chemical  systems,  as  well  as  the 
effect  of  all  physical  agents,  can  be  arranged  in  a  perfectly  general 
principle,  brought  forward  by  LeChatelier,  which  can  be  for- 
mulated as  follows: 

"  Every  external  influence  arouses  in  a  chemical  equilibrium 
system,  opposing  forces,  which,  after  the  ceasing  of  the  external 
forces,  strive  to  bring  it  back  to  its  original  condition." 

In  other  words  every  chemical  equilibrium  system  is  in  a 
certain  sense  to  be  conceived  of  as  elastic  ;  external  pressure  may 
have  an  influence  on  the  composition,  if  a  change  in  volume  is 
bound  up  with  the  chemical  reaction.  Raising  the  pressure 
displaces  the  composition  so  that  a  decrease  of  volume  must  take 
place,  if  the  pressure  ceases,  the  reaction  returns  in  the  reverse 
sense  and  halts  when  the  original  composition  is  reached. 

Since  in  the  equation 

Fe304+4H2  <=*  3 


with  the  exception  of  the  very  small  change  in  the  specific  volume 
of  the  solid  phase,  which  can  be  entirely  neglected,  a  difference 
of  volume  does  not  exist  between  the  beginning  and  end  con- 
dition, a  change  of  pressure  can  exert  no  influence  on  the  com- 
position of  this  system  in  equilibrium. 

The  equilibrium  between  the  metal,  oxide,  and  the  two 
gaseous  substances  depends  alone  on  the  composition  of  the  gas 


152  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

phase.  It  is  immaterial  whether  the  gas  is  under  a  pressure  of  a 
few  millimeters  or  many  atmospheres. 

If  the  composition  deviates  from  the  equilibrium  proportions 
a  reaction  takes  place,  with  an  excess  of  hydrogen  reduction  of 
the  oxide  with  an  excess  of  water  vapor  oxidation  to  ferroso- 
ferric  oxide.  The  principle  of  LeChatelier,  accordingly,  allows 
us  an  important  insight  into  the  interesting  oxidation  and 
reduction  processes. 

If  we  would  press  still  deeper  into  the  conditions  of  the  same 
so  we  must  consider  that  beside  the  hydrogen  and  the  water 
vapor  in  the  atmosphere  there  must  still  be  present  a  very  small 
amount  of  oxygen,  depending  on  the  decomposition  tension.  It 
is,  in  our  special  case,  so  small  that  it  escapes  detection  by  the 
analytical  methods,  but  its  assumption  gives  us  a  convenient 
means  of  investigating  the  equilibrium  conditions. 

The  Mass  Law. 

We  can  trace  back  the  process  of  the  reduction  of  the  oxide  to 
the  burning  of  hydrogen  in  oxygen,  which  is  subjected  to  the 
special  condition,  that  the  partial  pressure  corresponding  to  the 
equilibrium  between  the  gaseous  substances  hydrogen,  oxygen 
and  water  vapor  is  equal  to  the  oxygen  tension  of  the  oxide. 

The  burning  of  hydrogen  is  a  reversible  process;  at  high 
temperatures  water  breaks  down  into  its  elements  so  that  we  may 
write  the  equation 

2H2+02  ^  2H20. 

We  meet  here  a  reversible  reaction  between  pure  gaseous 
substances  which  are  all  simultaneously  present  in  the  homo- 
genous gas  phase  and  are  in  equilibrium  with  each  other. 
Reactions  in  homogenous  systems  in  gases  and  in  solutions  are 
best  studied  with  the  aid  of  the  mass  law,  formulated  by  the  two 
Norwegian  investigators,  Guldberg  and  Waage  which  declares 
"  The  intensity  of  a  chemical  action  is  proportional  to  the  con- 
centration of  the  reacting  molecular  species." 

If  the  two  substances  A  and  B  react  with  each  other  with  the 
formation  of  D  according  to  the  equation. 

A+B-+D, 


THE  METALLURGICAL  REACTIONS  153 

and  we  designate  with  C  the  concentration  (number  of  gram 
molecules  per  unit  of  space)  and  with  K  a  proportionality  factor, 
the  intensity  of  the  reaction  from  left  to  right  will  be 


If  we  consider  a  reaction  which  can  also  take  place  in  the 
reverse  sense,  it  follows  that  the  intensity  of  the  reversal  will  be 
given  by 


The  equilibrium  condition  is,  therefore,  characterized  by  the 
equality  of  the  intensities  of  the  two  reaction  directions 

Therefrom 

Cr*       vf 
a'^b -A.    _  jr 

~Cd ~K  = 

The  fraction  on  the  left  side  of  the  equation  has  at  constant 
temperature  in  case  of  equilibrium  always  the  same  value  K. 
This  constant  is  called  the  equilibrium  constant,  or,  if  the  process 
is  looked  upon  as  a  dissociation  of  Z),  dissociation  constant. 

If  we  return  now  to  our  example  we  have,  for  the  intensity 
of  the  water  formation  from  one  molecule  of  oxygen  and  two  of 
hydrogen 


for  the  water  dissociation, 


Therefrom  the  equilibrium  constant 

K    Co2'C2H2 
A=~™ > 

(-  H20 

and  further  that  the  equilibrium  mixture  is  richer  in  water  vapor 
the  higher  the  concentration  of  the  oxygen.  This  conclusion 
is  of  importance  if  we  think  of  our  homogeneous  system  set  in 
equilibrium  with  metal  and  metal  oxide.  By  this  contact  the 
oxygen  is  brought  to  a  constant  value  corresponding  to  the 
oxygen  tension  of  the  oxide. 


154  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

In  case  of  the  complete  equilibrium  with  all  the  substances 
together,  the  system  must  be  in  equilibrium  as  regards  oxygen 
and  the  two  other  gases  and  with  metal  and  oxide;  we  bring  now 
all  the  constants  on  one  side  and  obtain 


Co2C 


H2o 


\K_ 

^ 


We  see  therefrom  that  for  the  equilibrium  between  metal,  metal 
oxide,  water  vapor  and  hydrogen,  a  definite  ratio  between  the 
two  gases  is  required.  This  is  only  true  if  the  metal  is  always 
present  as  a  solid  or  liquid  phase;  with  volatile  metals  the  con- 
centration of  the  metal  vapor  also  enters  into  the  constant. 
Since  the  absolute  value  of  these  concentrations  and  therewith 
also  the  pressure  under  which  the  gas  stands  is  without  meaning, 
then  the  ratio  77  can  exist  under  different  pressures.  We,  accord- 
ingly, obtain  the  same  result  which  we  had  previously  deduced 
with  the  help  of  the  principle  of  LeChatelier.  Every  deviation 
of  the  gas  composition  from  the  ratio  77  requires  reaction,  if  the 
ratio  is  greater  than  77  a  reduction  of  the  oxide  takes  place,  if  it  is 
smaller  an  oxidation  of  metal  to  oxide. 

If  we  represent  graphically  the  conditions  for  oxidation  and 
reduction  in  our  system,  as  depending  on  the  pressure  and 
composition  of  the  gas  atmosphere  at  constant  temperature,  we 
obtain  (see  Fig.  93),  as  limits  for  the  oxidation  and  reduction 
fields,  the  geometrical  locus  of  all  gas  systems  which  are  in 
equilibrium  with  metal  and  oxide.  For  all  these  the  abscissa  x 
which  gives  the  number  of  hydrogen  molecules  (the  sum  of  the 
hydrogen  and  water  vapor  molecules  is  equal  to  i)  is  defined 
by  the  equation 

x 
=  77. 

I—  X 

If  x  is  constant,  the  curve  is  parallel  to  the  p  axis. 

The  equilibrium  constant  77  is,  as  we  have  above  deduced, 


THE  METALLURGICAL  REACTIONS 


155 


and  is  dependent  on  the  dissociation  constant  of  water  K,  and 
also  on  the  oxygen  tension  of  the  oxide.  If  we  compare  the  value 
of  rj  for  different  metals,  we  see  that  rj  grows  with  decrease  of 
the  decomposition  pressure  of  the  oxide,  that  the  amount  of 
hydrogen  in  the  equilibrium  ratio  for  a  noble  metal  need  be  very 
small,  for  a  base  metal  it  must  be  under  some  conditions  ex- 
traordinarily large.  This  agrees  with  the  experimental  fact 
that  noble  metals  are  not  attacked  by  water  vapor,  their  oxides 


Oxydation 


Reduction 


10$   20$    30$    40 


70$    80$    90$  100X, 


FIG.  93. 


are  reduced  by  the  smallest  addition  of  hydrogen,  and  that  with 
base  metals  the  reduction  of  oxides  with  hydrogen  is  difficult. 

If  more  than  one  oxide  is  known  for  a  metal,  we  obtain  a 
number  of  values  for  rj  and  in  the  diagram  a  number  of  parallels. 

Concerning  the  other  magnitude  on  which  the  constant  77 
depends,  the  dissociation  constant  of  water  vapor,  exact  experi- 
ments have  recently  been  carried  out  by  Nernst  and  v.  Warten- 
berg.*  The  dissociation  of  water  vapor  is  first  observable  at 
very  high  temperature.  It  is  determined  by  passing  the  water 
*  Z.  Physik.  Chem.,  56,  534  (1906). 


156 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


vapor  through  a  porcelain  pipette  heated  in  an  electric  furnace 
and  measuring  the  resulting  oxy-hydrogen  in  a  fine  eudiometer 
tube. 

For  temperatures  above  1800°  the  method  worked  out  by 
Lowenstein  *  is  used.  The  water  is  led  through  an  electrically 
heated  iridium  tube,  and  is  partially  decomposed  into  oxy- 
hydrogen  gas.  The  hydrogen  partial  pressure  in  the  same  is 
measured  in  by  means  of  an  evacuated  iridium  bulb  connected 
with  a  manometer  and  placed  in  the  reaction  space.  This  metal 
is  permeable  to  hydrogen  but  not  to  the  other  gases,  the  dif- 
fusion into  the  inside  proceeds  till  the  pressure  of  the  hydrogen 
gas  inside  the  bulb  is  equal  to  the  partial  pressure  of  the  hydrogen 
from  the  dissociation  of  the  water  vapor.  The  manometer 
determines,  accordingly,  the  hydrogen  pressure  directly.  From 
the  experiments  the  following  values  have  been  found.  Water 
vapor  at  i  atmosphere  pressure  undergoes  by  heating  to  the  tem- 
perature T  a  decomposition  of  X  per  cent. 


r°  c. 

T°  Abs. 

X  Per  Cent. 

K. 

1124 

1397 

0.0073 

2.312.10-13 

1207 

1480 

0.0189 

3.794.10-12 

1288 

1561 

0.034 

2.094.  lo-u 

1882 

2155 

1.18 

6.418.10-7 

1984  . 

2257 

1.77 

2.080.10-6 

The  dissociation  is  dependent  on  the  temperature,  and 
increases  with  increasing  temperature.  For  low  temperatures, 
the  degree  of  decomposition  is  so  small  that  one  can  no  longer 
measure  the  resulting  oxy-hydrogen  gas. 

Van't  Hoff's  Equation. 

Thermodynamic  methods  are  used  to  reckon  the  equilibrium 
which  cannot  be  experimentally  determined.  Van't  Hoff  has 
deduced  an  equation  which  connects  the  dissociation  constant 
with  the  reaction  heat  and  the  temperature.  It  is 


*  Z.  Physik.  Chem.,  54,  715  (1906). 


THE  METALLURGICAL  REACTIONS  157 

if  Q  is  the  heat  of  combustion  of  the  hydrogen.  Now,  however, 
the  reaction  heat  is  in  no  way  a  constant  but  changes  with  the 
temperature.  The  change  is  according  to  thermodynamics 
equal  to  the  difference  between  the  specific  heats  of  the  factors 
and  the  products.  These  latter  magnitudes  are  dependent  on 
the  temperature  and  they  have  recently  been  exactly  deter- 
mined by  Holborn  and  Henning  who  have  given  the  following 
relation  for  the  molecular  heats  of  the  gases  concerned  (molecular 
heat  =  product  of  the  molecular  weight  and  the  specific  heat  at 
constant  volume  C)  for  water  vapor. 

CP(H2O)  =  5.62-0.00077T 

The  molecular  heats  of  the  permanent  gases  oxygen  and  hydrogen 
are  equal  and 

C«,  =  4.68-o.ooo267\ 

From  this  it  follows  that  the  temperature  coefficient  of  the 
reaction  2H2-fO2  =  2H2O  is 


-  2C,(H20)  =  2.8o-o.ooo76r. 


The  value  of  the  heat  of  combustion  at  100°  and  constant  volume 
to  form  two  molecules  of  water  is  Qioo=  11,500  cal.  The  tem- 
perature function  is 


If  we  substitute  this  value  in  the  above  differential  equation  and 
integrate  it  follows  if  we  replace  natural  logarithms  with  Briggsian 

log  tf'  =  log  £-25,030--1.40  log      -o. 


If  one  value  of  K  has  been  determined  experimentally  we  can, 
with  the  help  of  this  equation,  arrive  at  the  value  of  K  for  every 
other  temperature.  From  this  the  per  cent  of  decomposed 
water  vapor  (X)  can  be  calculated  at  every  other  gas  pressure, 


158 


THE   PHYSICAL  CHEMISTRY  OF  THE  METALS 


from  the  mass  law;  the  values  given  in  the  following  table  have 
been  arrived  at  in  this  way : 


T  Abs. 

X  Per  Cent. 

K. 

T  Abs. 

X  Per  Cent 

K. 

1000 

3.00.10-5 

2.  245.  IO-2O 

1800 

0.199 

3.628.  10-9 

IIOO 

I  .  82  .  10-4 

4.5l6.IO-l8 

1900 

0-354 

1.937.10-8 

1  200 

8.13.10-4 

3.693.10-16 

2000 

0.588 

8.461.10-8 

1300 

2.91.10-3 

1.564.10-14 

2IOO 

0-935 

3.251.10-7 

1400 

8.61.  10-3 

3.763.10-13 

22OO 

1.42 

i  .092.  10-6 

1500 

2.  21.  10-2 

5.944.IO-I2 

2300 

2.08 

3.311.10-6 

1600 

5,07.10-2 

6.733.IO-II 

2400 

2.92 

8.847.10-6 

1700 

0.105 

5.633.IO-IO 

2500 

3.98 

2.225.10-5 

TAbs. 

P  =  io  Atm. 
X  Per  Cent. 

P=i  Atm. 
X  Per  Cent. 

P=o.i  Atm. 
X  Per  Cent. 

P  =0.0  1  Atm. 
X  Per  Cent. 

1000 

I.4Q.IO-5 

3.000.10-5 

6.46.  10-5 

1.39.10-4 

1500 

I.03.IO-2 

2.  21.  10-2 

4.76.10-2 

0.103 

2000 

0.273  . 

0.588 

1.26 

2.70 

2500 

I.98 

3.98 

8.16 

16.6 

From  the  differential  equation 


it  can  be  at  once  deduced  whether  K  increases  or  decreases  with 
rising  temperature.  If  Q  is  positive,  that  is,  during  the  dissocia- 
tion heat  is  supplied,  K  increases  with  increasing  temperature. 
If  the  process  goes  on  with  heat  evolution,  if  heat  is  given  to  the 
surroundings  from  the  system,  the  differential  quotient  is  nega- 
tive and  K  is  smaller  with  rising  temperature. 

"  The  equilibrium  of  a  chemical  system  is  displaced  by  raising 
the  temperature  in  the  sense  that  the  product  resulting  from  heat 
absorption  is  favored." 

This  law  can  be  derived  from  LeChatelier's  principle,  since 
also  in  thermal  relations  chemical  equilibrium  systems  are 
elastic,  and  by  cooling  the  equilibrium  swings  back  and  liberates 
the  amount  of  heat  which  has  been  added. 

We  are  now  oriented  concerning  the  constant  K,  for  the 
reaction  between  hydrogen  and  oxygen.  If  we  combine  it  in 


THE  METALLURGICAL  REACTIONS 


159 


the  above-named  way  with  the  oxygen  dissociation  pressure  of 
oxides,  we  are,  able  to  obtain  mathematically  the  equilibrium 
ratio  rj  for  all  these  oxides. 

In  practice  there  comes  into  consideration  only  the  reaction 
with  which  we  started  and  to  which  we  now  return. 


Fe304+4H2  <=*  3 

The  equilibrium  has  been  investigated  by  Deville  *  and  more 
recently  by  G.  Preuner.f  We  give  in  the  following  table  the 
values  which  appear  to  be  the  nearest  correct. 


Temperature 
in  Degrees. 

CH2O 

77  ~    CH2 

Per  Cent  H2 

Observer. 

2OO 

20.41 

95-32 

265 

14.49 

93-56 

1 

360 
440 

8.405 
5-682 

89.39 
85-06 

Deville,  Liebig's  Ann.,  157,  71  (1872). 

770 

-852 

64.94 

Q2O 

•515 

60.23 

900 

•  449 

59-27 

' 

1025 

.282 

56.18 

Preuner,  Physik.  Chem.,  47,  416  (1904) 

1150 

163 

53.76 

J 

The  value  of  t\  decreases  with  rising  temperature;  we  may 
draw  the  conclusion  therefrom  that  the  reduction  of  FesC^  goes 
on  with  heat  absorption. 

Reduction  by  Cat bon  Monoxide. 

The  second  gaseous  reducing  agent,  carbon-monoxide,  plays 
in  practice  .a  still  greater  role  than  hydrogen.  In  general  there 
exists  between  the  two  reducing  gases  close  agreement  in  proper- 
ties. In  the  treatment  of  oxides  with  carbon  monoxide  we  meet 
a  reversible  action  precisely  as  in  the  treatment  with  hydrogen. 
This  can  be  represented  by  the  general  scheme. 

MeO+CO  <=±  Me+C02. 

This  decomposition  also  takes  place  without  change  of  volume; 

the  reaction  products  occupy  the  same  space  as  the  factors. 

*  Ann.,  157,  71  (1872).  f  Z.  Physik.  Chem.,  47,  416  (1904). 


160  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  reaction  is  independent  of  the  pressure  of  the  gas  and  comes 
to  rest  when  a  definite  ratio  between  carbon  monoxide  and  diox- 
ide exists;  which  ratio  is  dependent  on  the  temperature.*  This 
equilibrium  ratio 


= 

Cco2     *     \'Co2' 

is  dependent,  as  the  corresponding  magnitude  for  hydrogen,  on 
the  oxygen  pressure  of  the  oxide  to  be  reduced  and  the  dissoci- 
ation constant  of  C02,  K',  which  is  defined  by  the  equation 

~,     C2co-Co2 

"-- 


Then  also  the  combustion  of  CO  into  C02  is  a  reversible  reaction. 
Carbon  dioxide  decomposes  at  high  temperature  into  CO  and  O2. 
This  can  be  represented  by  the  equation 

2CO+O2  <=>  2C02. 

The  equilibrium  ratio  of  this  system  has  been  studied  by 
Nernst  and  v.  Wartenberg,  by  the  same  method  as  was  described 
for  the  dissociation  of  water  vapor.  They  observed  the  following 

*  This  again  is  only  true  for  non-volatile  metals. 
If  we  consider,  for  example,  the  system 

ZnO+CO«=±Zn+CO2, 

we  find  that  since  at  temperatures  where  the  reaction  goes  on,  zinc  only  exists 
as  a  vapor,  the  ratio  of  CO  to  COz  is  therefore  not  dependent  only  on  the  tem- 
perature but  the  equilibrium  constant  for  the  system  is  given  by 

ZnXCO2_ir 
~CO~ 

and  hence  the  concentration  of  zinc  vapor  must  be  taken  into  account.  In  the 
various  unsatisfactory  investigations  which  have  been  made  of  the  zinc  equilibrium 
this  has  not  been  taken  into  account.  See,  for  example,  the  recent  paper  by 
Fulton,  Bull.  Am.  Inst.  Min.  Eng.,  140,  1375  (1918);  also  the  earlier  paper  of 
Lencauchez,  Mem.  Soc.  Ing.  Civils,  1877,  568;  Eng.  Min.  J.,  26,  n  (1878). 


THE  METALLURGICAL  REACTIONS 


161 


decomposition  values,  x-per  cent  decomposed  CO2  at  the  given 
temperatures 


T  abs. 
x  per  cent 


1300 
0.00414 


1400 

O.OI— O.O2 


1478 
0.029-0.035 


From  these  values  and  the  thermal  data  the  heat  of  combus- 
tion of  CO,  6800  cal.  (at  constant  pressure)  the  molecular  heat 
of  C02. 


and  for  the  permanent  gases  CO  and  O2 


the  dissociation  constant  K  and  the  per  cent  of  decomposed  gas 
x  for  different  temperatures  and  pressures,  is  calculated  in  a 
similar  way  as  for  the  H2O  dissociation. 


Tabs. 
Degrees. 

X  Per  Cent. 

K'. 

Tabs. 
Degrees. 

X  Per  Cent. 

K  Pressure  I 
Atm. 

1000 

1.58.10-5 

3.  280.  IO-2I 

1800 

0.507 

6.016.10-8 

IIOO 

2.00.10-4 

5.99I.IO-I8 

1900 

0.978 

4.III.  10-7 

1200 

8.94.  10-4 

4.908.10-16 

2000 

1.77 

2.335.10-6 

1300 

3.89.10-3 

3.735.IO-I4 

2IOO 

3-03 

1.130.10-5 

I4OO 

1.38.10-2 

I.550.IO-I2 

2200 

4.88 

4.595.10-5 

1500 

4.06.  IO-2 

3.684.  io-ii 

2300 

7-55 

1.675.10-4 

I600 

0.104 

5.813.  10-10 

24OO 

11.3 

5.631.10-1 

I7OO 

o.  242 

6.905.10-9 

2500 

15-8 

1-552 

Tabs. 
Degrees 

10  At. 

i  At. 

o.i  At. 

o.oi  At.  =Pressu:e. 

IOOO 

7.31.10-6 

1.58 

3-40 

7-31=*% 

1500 

1.88 

4.06 

8.72 

0.188 

2OOO 

0.818 

1.77 

3-73 

7.88 

25OO 

7.08 

15-8 

30-7 

53-o 

The  equilibrium  ratio  has  been  determined  for  the  system  iron 
oxide,  iron,  carbon  monoxide,  carbon  dioxide.     The  reaction 


FeO+CO<=±Fe+CO2, 


162 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


gives  the  following  values : 


Temperature- 
Degrees 

CO,  Per  Cent. 

CO2,  Per  Cent. 

CO 
C0"2=77- 

Observer. 

552 

53-7 

46.3 

.160 

556 

53-4 

46.6 

.  146 

Schenck, 

561 

'        53-6 

46.4 

•  155 

* 

596 

55-5 

44-  S 

.247 

Semiller  and 

6lp 

56.8 

43-2 

•  315 

65I 

57-9 

42.1 

•375 

Falcke 

662 

58.4 

41.6 

.404 

The  equilibrium  constant  increases  with  increasing  tempera- 
ture, that  is,  the  left  side  of  the  reaction  is  favored  as  the  reduc- 
tion takes  place  with  heat  evolution.  The  reaction 

Fe304+CO  «=>  3FeO+C02, 

also  has  a  measureable  equilibrium  which  has  been  determined 
by  Baur  and  Glasner.*  They  found  at  581°,  41.9  per  cent 
CO  and  57.4  per  cent  CO2  corresponding  to  77  =  0.721. 

The  reversibility  of  the  action  of  CO  on  iron  oxide,  the  oxida- 
tion of  metal  by  C02  is  of  the  greatest  importance  in  the  blast 
furnace  process  which  we  shall  consider  in  the  next  chapter.  It 
gives  us  the  explanation  of  the  remarkable  fact  that  the  carbon 
monoxide  used  as  a  reducing  agent  does  not  go  completely  to 
CO2  but  occurs  in  rich  amounts  in  the  exit  gases  of  the  blast 
furnace  and  becomes  a  valuable  by-product. 

We  have  become  familiar  with  the  characteristics  of  the  two 
gaseous  reducing  agents  CO  and  H2.  We  will  find  which  is  the 
strongest  reducing  agent.  The  question  is  now  easily  answered 
since  we  are  familiar  with  the  dissociation  ratios  of  the  oxidation 
products  of  H2  and  CO.  The  strongest  reducing  agent  is  the 
substance  whose  oxidation  product  is  the  least  dissociated,  that 
is,  whose  dissociation  constant  possesses  the  smallest  value.  In 
order  to  make  clear  the  comparative  ratios  of  the  two  reducing 
gases  we  plot  the  value  of  X  or  K  for  water  vapor  and  carbon 
dioxide  or  better  their  logarithms  against  temperature,  we 

*  Z.  Physik.  Chem.,  43,  354  (1903). 


THE  METALLURGICAL  REACTIONS 


163 


obtain  two  curves  (Fig.  94).  These  cut  at  a  temperature  of 
1140°  abs.  or  867°  C.  Here  the  H20  and  C02  are  equally 
strongly  dissociated,  hydrogen  and  CO  are  equally  strong  reduc- 
ing agents,  beneath  this  temperature  the  CO  reduces  more  ener- 
getically; above  it  the  reduction  is  more  active  with  hydrogen. 

This,  however,  does  not  exhaust  what  we  have  to  say  con- 
cerning CO.  The  gas  undergoes,  under  definite  conditions,  a 
charateristic  decomposition  giving  as  the  products  C02  and  solid 
C.  This  reaction  which  was  first  observed  by  St.  Clair  Deville  * 


Dissociation  of  C  02  and  H20  Pressure  1  Atm. 
x    per  cent  decomposed  Gas 


1000°  1200°  1400°  1600°  1800°  2000°  2200°  2400°  2600°  Temp.abs 
FlG.  94. 

does  not  proceed  alone  but  requires  the  presence  of  certain  con- 
tact materials.  The  metals  of  the  iron  group  Ni,  Co,  Pe  and  Mn, 
are  especially  active  in  starting  the  decomposition  process  and, 
indeed,  they  do  this  without  being  thereby  changed.  Their 
action  is  the  greater,  the  greater  the  surface.  We  are  familiar 
with  the  reverse  of  the  above  process.  The  method  by  which  CO 
is  obtained  by  leading  CO2  over  glowing  C  is  a  very  useful  one 
and  plays  an  important  role  in  the  formation  of  producer  gas. 
The  reaction 


Compt.  rend.,  59,  873  (1854). 


164  THE   PHYSICAL   CHEMISTRY  OF  THE  METALS 

is  accordingly  reversible,  and  runs  to  completion  in  neither 
direction  but  comes  to  equilibrium  between  the  concerned 
substances.  Concerning  this  equilibrium  of  two  components 
C  and  0  divided  between  two  phases  the  solid  C  and  the  gas 
mixture,  the  phase  rule  says  that  it  possesses  a  two-degree  free- 
dom, that  is,  the  two  phases  can  exist  together  under  different 
temperatures  and  pressures. 

Concerning  the  direction  in  which  the  composition  of  the  gas 
is  changed  by  change  of  pressure,  LeChatelier's  rule  says, 
through  raising  the  pressure  that  reaction  is  favored  which  goes 
on  with  a  decrease  of  volume.  It  is  the  decomposition  of  CO 
since  two  volumes  of  this  gas  give  one  volume  of  CCb,  the  simul- 
taneously formed  solid  substance  is  neglected  in  comparison  to 
the  gas.  The  CO  can  accordingly  be  decomposed  in  the  pres- 
ence of  C  at  a  constant  temperature  by  compressing  the  gaseous 
atmosphere.*  By  lowering  the  pressure  the  C02  acts  on  the 
carbon  and  forms  the  substance  with  the  greater  volume,  that 
is  the  CO  again  (naturally  these  reactions  take  place  more  quickly 
in  the  presence  of  a  contact  substance).  At  constant  tempera- 
ture every  gas  pressure  corresponds  to  an  entirely  definite  ratio 
of  the  two  gaseous  oxides. 

The  dependence  of  this  ratio  on  pressure  can  be  determined 
from  the  law  of  mass  action.  We  consider  again  the  reaction  in 
the  gas  phase  and  assume  that  a  small  immeasurable  quantity 
of  carbon  vapor  is  in  it.  In  a  similar  way  as  above  for  the  dis- 
sociation process,  the  relations  for  equilibrium  can  be  devel- 
oped. 

C2co 


Cc-Cco2 


=  Const. 


The  concentration  of  carbon  vapor  that  is  in  equilibrium  with  the 
solid  phase  is,  at  constant  temperature,  a  constant  magnitude, 

*  For  the  effect  of  pressure  on  the  reaction 

2CO  *=>  C+CO2, 

see  Rhead  and  Wheeler,  Trans.  Chem.  Soc.,  97,  2181  (1910).  These  investigators 
find  a  very  close  agreement  between  the  effect  of  pressure  as  experimentally  de- 
termined and  as  calculated  from  the  mass  law. 


THE  METALLURGICAL  REACTIONS  165 

depending  on  the  vapor  pressure  of  the  carbon.  If  we  combine 
this  constant  with  the  constants  on  the  right  side  and  designate 
the  magnitude  obtained  by  £  we  obtain  as  the  equilibrium  rela- 
tion for  the  two  oxides  of  carbon,  if  they  are  in  contact  with  solid 
C  the  equation 

C2co 
r    Cco2* 

We  will  now  make  the  assumption  which  we  have  previously 
employed  that  the  sum  of  the  partial  pressures  of  the  two  oxides 
is  constant;  we  set  this  equal  to  i.  If  the  number  of  monoxide 
molecules  present  is  x  the  dioxide  molecules  are  i  —  x.  If  we  set 
the  total  pressure  of  the  two  gases  equal  to  P  the  partial  pressures 
are  x-P  and  (i—  x)  P  respectively.  The  concentrations  of  the 
two  gases  are  proportional  to  these  magnitudes.  If  one  substi- 
tutes them  in  the  equation  it  follows: 


or      - 


I—X  X2 

These  equations  give  us  the  relation  between  the  composi- 
tion of  the  gas  atmosphere  (x.ioo  is  the  CO  per  cent)  and  the 
pressure  under  which  it  stands  in  case  there  is  equilibrium 
between  the  substances  present. 

In  the  discussion  of  the  equations  it  is  brought  out  that  x 
increases  with  rising  values  of  P.  The  requirements  of  Le- 
Chatelier's  principle  which  we  have  previously  spoken  of  are 
accordingly  fulfilled.  To  get  a  conception  of  the  existing  rela- 
tions we  will  represent  them  graphically.  The  given  equation 
represents  a  curve  of  the  third  degree.  Its  position  is  shown  in 
Figs.  95  and  950.  Since  in  our  case  the  abscissa  x  cannot  be 
greater  than  i,  only  that  part  of  the  curve  with  the  abscissa 
between  O  and  i  has  a  real  meaning.  For  later  consideration  it 
is  well  to  give  here  its  further  extent.  In  Fig.  950  the  real  part 
is  drawn.  For  x  —  2  the  curve  reaches  a  minimum,  with  increas- 
ing x  it  passes  through  a  point  of  inflection  and  approaches  the 
abscissa  axis  asymptotically  intersecting  it  at  x  =  o.  We  have, 
accordingly,  a  cubic  hyperbola.  This  form  of  curve  is  met  with  in 


166 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


many  homogeneous  equilibrium   systems,  for  example,   in  all 
binary  dissociation  processes  it  has  considerable  importance. 


%co 


FIG.  95. 


FIG.  950. 

The  curve  which  represents  the  geometrical  locus  of  all  pos- 
sible mixtures  of  monoxide  and  dioxide  that  are  in  equilibrium 
with  solid  carbon,  is  accordingly  an  isotherm  dividing  the  field 


THE  METALLURGICAL  REACTIONS  167 

(Fig.  95)  in  two  halves.  In  these,  reactions  take  place,  in  the 
right  decomposition  and  in  the  left  formation  of  CO. 

We  find  in  the  drawing  still  another  curve  (dotted)  which 
begins  at  the  point  x,=  i  and  P  =  Po  and  terminates  at  x  =  o 

and  P  =  —  it  is  the  reaction  path  of  the  decomposing  of  CO, 

2 

i.e.,  the  series  of  points  through  which  the  decomposing  CO  of 
original  pressure  PQ  passes  during  its  decomposition.  If  we 
carry  the  reaction  entirely  to  the  end  the  complete  transforma- 
tion into  carbon  dioxide  is  reached  when  the  gas  pressure  has 
become  half  of  the  original  pressure.  The  form  of  this  reaction 
path  is  indicated;  the  partial  pressure  of  the  still  present  monox- 
ide in  the  decomposing  gas  p  stands  in  the  following  relation  to 
the  original  pressure  PQ  and  the  observed  total  pressure  P: 


then  the  observed  difference  from  the  original  pressure  repre- 
sents the  double  difference  of  the  decomposing  CO. 
Now    =  x-P  and  thence  follows 


This  equation  is  represented  geometrically  by  a  hyperbola. 
If  we  consider  P  as  variable  we  obtain  a  series  of  hyperbolas  for 
which  P  is  infinite  for  the  case  x  =  2. 

The  reaction  path  is  also  a  section  of  a  hyperbola.  This 
usually  reaches  its  end  as  the  equilibrium  between  the  finely 
divided  carbon  and  its  oxides  is  reached,  i.e.,  in  the  intersection 
of  the  equilibrium  curve  and  the  reaction  path. 

The  equilibrium  between  carbon  and  its  oxides  is  now,  as 
practical  experience  in  the  preparation  of  generator  gas  has  also 
shown,  strongly  dependent  on  the  temperature  and  at  high 
temperatures  very  much  more  monoxide  is  observed  than  at  low 
temperatures.  From  this  we  draw  the  conclusion  that  the  forma- 
tion of  carbon  monoxide  is  a  process  which  goes  on  with  heat 
absorption.  Correspondingly  heat  is  evolved  in  the  decomposi- 
tion of  the  monoxide,  the  thermochemistry  of  the  decomposition 
reaction  is  expressed  by  the  following  equation. 
2CO=C+C02-39,ooo  cal. 


168 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


there  is  accordingly  a  very  considerable  amount  of  energy  freed 
in  the  form  of  heat. 

With  increasing  temperature  the  equilibrium  constant  also 
increases.  Its  change  with  the  temperature  can  be  gotten  from 
measurements  of  Boudouard  *  on  the  composition  of  monoxide 
dioxide  mixtures,  that  are  in  equilibrium  with  carbon  at  atmos- 
pheric pressure  and  different  temperatures.  The  following 
table  gives  the  values  found  from  the  observations  and  thermo- 


1100 
1000° 

900° 
800° 
700° 
600° 
500C 


2  CO±^ 

Pressure  I  Atmosphere 


90      100$  CO 


0         10        20        30        40        50        60        70        I 
FIG.  96. 

dynamic  calculations.     The  dependence  on  temperature  is  shown 
still  better  by  the  graphical  representation  of  the  results.  (Fig.  96) 


Temperature, 
Degrees. 

Co,  Per  Cent. 

C0«,  Per  Cent. 

Temperature, 
Degrees. 

CO,  Per  Cent. 

COj,  Per  Cent. 

450 

2 

98 

800 

90 

IO 

500 

5 

95 

850 

94 

6 

550 

ii 

89 

900 

96.5 

3-5 

600 

23 

77 

925 

97 

3-0 

650 

39 

61 

950 

98.5 

i-5 

700 

68 

32 

IOOO 

99-3 

0.7 

750 

76 

24 

1050 

99.6 

0.4 

*  Ann.  Chim.  Phys.,  (7),  24,  5  (1901). 

More  recent  and  accurate  measurements  of  this  equilibrium  have  been  made 
by  Rhead  and  Wheeler,  Trans.  Chem.  Soc.,  97,  2181  (1910).  For  a  discussion 
of  the  results  of  these  investigators  from  the  standpoint  of  thermodynamics  see 
Lewis  and  Randall,  J.  Am.  Chem.  Soc.,  37,  458(1915). 


THE  METALLURGICAL  REACTIONS 


169 


The  values  of  f  vary  still  further  with  the  different  modifica- 
tions of  carbon.  Graphite  gives  different  results  from  amorphous, 
wood  or  sugar  carbon,  or  the  modification  resulting  from  the 
decomposition  of  the  monoxide.  The  ratio  between  these 
equilibrium  constants  can  be  determined  in  a  way  which  we 
shall  consider  later.  Schenck  and  Heller  *  found  the  following 
numbers : 


Temperature, 
Degrees. 

Graphite. 

CO-carbon. 

Sugar  Carbon. 

600 
660 

I 
I 

5 
6 

5-5 
6.6 

It  is  accordingly  not  immaterial  in  the  preparation  of  gener- 
ator gas  whether  a  graphite  like  coal  is  used  or  an  amorphous 
form,  the  latter  is  to  be  desired  since  it  gives  gas  richer  in  a  carbon 
monoxide  under  the  same  conditions. 

These  relations  can  be  at  once  seen  if  we  recall  the  develop- 
ment of  £  with  the  help  of  the  mass  law.  We  see  that  the  con- 
centration of  carbon  vapor  which  is  in  equilibrium  with  the  car- 
bon enters  into  these  magnitudes.  This  carbon  vapor  pressure 
is  different  for  the  different  carbon  modifications.  We  can  get 
from  this  as  Smits  f  has  done  that  the  relations  between  the 
values  of  f  are  the  direct  relations  between  the  vapor  pressures 
of  the  different  carbon  modifications. 

We  have  considered  rather  fully  the  relations  of  carbon 
monoxide.  We  must,  however,  study  its  peculiarities,  its  rela- 
tion to  carbon  dioxide  on  the  one  hand,  and  to  carbon  on  the 
other  since  in  all  reduction  with  the  help  of  carbon  this  oxide 
occurs  as  a  reaction  product  and  frequently  influences  the  course 
of  the  reaction. 


Ber.,  38,  2139  (1905). 


t  Ber.,  38,  4027  (1905). 


CHAPTER  V 

DECOMPOSITION     OF     CARBON     MONOXIDE,    BLAST- 
FURNACE  PROCESSES 

We  have  already,  in  a  previous  chapter,  learned  of  the 
remarkable  decomposition  which  carbon  monoxide  suffers  under 
certain  conditions,  namely,  its  decomposition  into  carbon  dioxide 
and  elementary  carbon.  We  have  seen  that  this  process  can 
only  take  place  in  the  presence  of  contact  material  and  that  the 
finely  divided  metals  of  the  iron  group,  nickel,  cobalt  iron  and 
manganese  serve  as  such.  We  have  also  determined  the  reac- 
tion path  of  the  decomposing  monoxide  and  the  place  at  which 
th'e  decomposition  halts. 

Catalytic  Decomposition  of  Carbon  Monoxide. 

All  these  theoretical  conclusions  are  confirmed  if  the  decom- 
position is  carried  through  at  a  constant  temperature,  near  the 
boiling  point  of  sulfur,  using  as  a  catalyst  finely  divided  nickel 
or  cobalt.  Fig.  97  shows  the  change  of  pressure  with  time,  of  a 
given  amount  of  carbon  monoxide  at  constant  volume,  during 
the  decomposition  reaction  in  the  presence  of  nickel  and  iron. 
This  has  been  observed  by  Schenck  and  Zimmermann  in  their 
investigation  on  the  decomposition  of  carbon  monoxide.  As  is 
seen,  there  is  a  considerable  difference  in  the  action  of  the  two 
metals.  With  nickel  the  reaction  comes  to  a  standstill,  when 
the  equilibrium  between  carbon  and  its  two  gaseous  oxides  is 
reached  as  we  should  expect  from  our  earlier  experiences.  The 
final  pressure  reached  should,  therefore,  not  be  under  half  of  the 
original  pressure  of  the  pure  carbon  monoxide. 

To  our  great  astonishment,  we  meet  with  iron  an  entirely 
different  condition,  a  very  large  pressure  decrease,  that  we  can 

170 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


171 


only  explain  by  a  transition  of  the  gas  into  a  solid  product. 
Sometimes  the  final  pressure  is  only  a  small  per  cent  of  the  orig- 
inal pressure.  If  we  investigate  the  contact  mass  more  care- 
fully it  is  found  that  along  with  the  precipitation  of  carbon  an 
oxidation  of  metallic  iron  has  taken  place. 

We  encounter  the  paradox,  that  iron  is  oxidized  by  the  strong 
reducing  agent,  pure  carbon  monoxide.  At  first  sight  this  fact 
appears  contrary  to  all  we  have  learned  in  the  previous  lecture. 

i 
Pressure  m  m. 


Nickel  445' 


50   100   150   .200   250   300   350   400   450   500 

.Minutes 

FIG.  97. 

The  entire  problem  is  apparently  thereby  complicated  since  with 
other  original  pressures  the  decomposition  goes  as  in  the  pres- 
ence of  cobalt  and  nickel. 

The  solution  of  the  problem  is  reached  when  one  thinks 
that  from  the  decomposition  of  carbon  monoxide,  the  dioxide 
results,  and  that  its  concentration  can,  under  some  conditions, 
grow  so  great  that  the  equilibrium  ratio  of  the  two  gases  with 
iron  and  iron  oxide  is  exceeded.  It  then  follows  that  the 
metal  is  oxidized  with  the  formation  of  carbon  monoxide,  and, 
on  the  other  hand,  this  gas  decomposes  again  with  the  precipita- 
tion of  carbon.  By  the  alternation  of  the  two  processes,  oxi- 


172 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


dation  of  the  metal  and  the  decomposition  of  the  carbon  monox- 
ide, all  the  carbon  as  well  as  all  the  oxygen  can  go  over  to  the 
solid  state.  The  reaction  path  (see  page  133  and  Fig.  98),  is 
under  these  conditions  complicated  further  than  with  the  simple 
decomposition  in  the  presence  of  nickel.  If  we  assume  the  same 
original  pressure  and  temperature  for  the  reaction  in  the  presence 
of  iron  and  of  nickel  then  the  two  hyperbola  limbs  fall  together 
until  the  ratio  of  the  two  gaseous  oxides  of  carbon  has  reached  a 
value  at  which  oxidation  of  the  iron  results.  In  the  presence  of 
nickel,  which  is  not  oxidized,  the  reaction  proceeds  along  the 


•%CO 


FIG.  98. 


hyperbola  till  the  curve  is  reached  which  represents  the  equilib- 
rium between  carbon  and  its  oxides.  In  the  presence  of  iron 
there  is  a  break  in  the  reaction  path,  the  pressure  decreases 
strongly  while  the  composition  of  the  gases  approaches  a  value  at 
which  it  is  in  equilibrium  with  carbon  and  with  iron  and  iron 
oxide.  We  will  see  further  on,  that  it  depends  on  the  position 
of  this  total  equilibrium  and  on  the  original  pressure  of  the  carbon 
monoxide  whether  an  oxidation  of  the  metal  by  the  gas  takes 
place  or  whether  the  metal  remains  unoxidized  and  acts  as  con- 
tact material  for  the  decomposition  of  carbon  monoxide  into 
carbon  dioxide  and  solid  carbon  as  nickel  does. 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC.  173 

Equilibrium  Between  Carbon  Monoxide  and  Iron. 

The  total  equilibrium  between  iron,  iron  oxide,  carbon  and  the 
two  gaseous  oxides,  carbon  monoxide  and  carbon  dioxide  is  rep- 
resented as  an  equilibrium  of  a  three-component  system  (com- 
ponents, carbon,  iron  and  oxygen)  with  the  simultaneous  pres- 
ence of  four  phases  (Fe,  FeO,  C,  gas).  According  to  the  phase 
rule  such  a  system  must  possess  one  degree  of  freedom,  the 
equilibrium  is  univariant  precisely  as  a  vapor  pressure.  Every 
temperature  corresponds  to  an  entirely  definite  equilibrium 
pressure. 

We  arrive  at  the  same  results  by  the  use  of  the  mass  law. 
We  have  for  our  case,  the  equilibrium  conditions  for  two  bivari- 
ant  systems 

1.  FeO+CO<=>Fe+C02, 

2.  2CO<=±C+C02. 

that  must  be  simultaneously  fulfilled.     The  first  may,  as  we  have 
seen  earlier,  be  expressed  by  the  equation 


and  second  by 

x2 

2.  — 

i-x 


If  we  eliminate  x  from  these  two  simultaneous  equations  we 
obtain  an  expression  for  the  total  pressure  of  the  two  gases  P 
in  which  this  magnitude  depends  only  on  the  equilibrium  con- 
stants t]  and 


It  follows  from  this  so  long  as  f  and  TJ  are  constants  for  a  given 
temperature,  that  every  temperature  corresponds  to  an  entirely 
definite  gas  pressure,  a  definite  sum  for  the  partial  pressures. 
From  Eq.  (i)  it  follows  further  that  also  the  composition  of  the 
gas  phase  is  absolutely  fixed. 

The  facts  can  be  made  very  plain  if  we  show  the  equations 
graphically. 


174  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

We  have  already  recognized  a  parallel  to  the  ordinate  axis  as 
a  geometrical  locus  of  the  equilibrium  between  metal,  oxide  and 
the  two  gases,  and  a  cubic  hyperbola  as  the  geometrical  ex- 
pression of  the  equilibrium  between  carbon  and  its  oxides.  The 
two  intersect,  and  at  the  intersection,  the  two  equilibria  are 
simultaneous,  it  represents  the  total  equilibrium  between  metal, 
metal  oxide,  carbon  and  the  two  gaseous  oxides  (see  Fig.  98). 
The  two  equilibrium  lines,  straight  and  cubic  hyperbola  divide 
the  plane  of  our  picture  into  four  fields,  in  these  the  following 
reactions  take  place: 

Field     I.  2CO-+C+C02 

FeO+CO->Fe+C02 
Field    II.  2CO-»C+C02 

Fe+C02-»Fe+CO 
Field  III.     C+C02-»2CO 

Fe+C02->Fe+CO 
Field  IV.     C+CO2-+2CO 

FeO+CO-*Fe+C02 

The  reduction  of  iron  oxide  or  generally  of  oxide  to  metal 
accordingly  goes  on  only  in  fields  I  and  IV.  Carbon  monoxide  can 
act  as  a  reducing  agent  inside  of  both.  Solid  carbon  can  only 
act  as  a  direct  reducing  agent  in  field  IV.  In  field  II  the  two  sub- 
stances carbon  and  oxide,  remain  together,  in  field  III  the  carbon 
gradually  disappears  without  thereby  attacking  the  metal  oxide. 

Of  all  points  of  the  P—X  diagram  the  solid  phases,  metal 
oxide  and  carbon,  are  only  present  at  the  point  of  total  equilib- 
rium, the  intersection  of  the  curve  with  the  line. 

In  which  way  this  occurs  we  have  already  seen  in  the  consider- 
ation of  the  decomposition  of  carbon  monoxide  in  the  presence 
of  iron.  We  have  there  confined  ourselves  to  a  special  case ;  we 
have  already  mentioned,  however,  that  the  decomposition  of  the 
pure  carbon  monoxide  in  the  presence  of  iron  can  take  place  so 
that  no  oxidation  of  metal  results.  Now  we  will  determine  the 
conditions  under  which  the  two  forms  of  carbon  monoxide  decom- 
position take  place. 


DECOMPOSITION  OF  CARBON  MONOXIDE,   ETC.  175 

The  original  pressure  of  the  pure  carbon  monoxide  is  decisive 
for  this,  on  it  depends  whether  the  hyperbolic  reaction  path  (see 
above)  lies  above  or  below  the  intersection  of  the  cubic  curve, 
with  the  line.  We  have  learned  of  the  first  case  above,  the  reac- 
tion path  meets  first  the  line  and  suffers  a  deflection  going  into 
field  I  and  field  II  where  oxidation  of  the  metal  results.  If  the 
hyperbola  runs  beneath  the  equilibrium  point,  so  it  strikes  the 
decomposition  curve  first,  and  with  the  attainment  of  the  simple 
carbon  equilibrium  the  reaction  comes  to  rest,  since  inside  of 
fields  I  and  IV  the  oxidation  of  the  metal  is  impossible. 

The  limiting  case  between  the  two  is  given  if  the  reaction  path 
cuts  directly  at  the  point.  It  can  be  easily  calculated  what  the 
original  pressure  of  CO  was  when  this  condition  is  satisfied.  The 
equation  of  this  hyperbolic  reaction  path  has  been  derived  in  the 
fourth  chapter.  It  is 

P0  =  (2-X)P. 

If  we  substitute  in  this  equation  the  ordinates  of  the  total 
equilibrium,  namely, 


and 

" 


*=. 


it  follows  for  the  limiting  case: 

Ma-^)f£?~r 

If  the  original  pressure  of  the  carbon  monoxide  is  greater  than 
this  value,  the  metal  is  itself  finally  oxidized,  if  it  is  smaller  the 
metal  acts  only  as  contact  substance. 

The  position  of  the  equilibrium  point  and  the  magnitude  of  the 
equilibrium  pressure  are  dependent  on  the  nature  of  the  metal, 
the  carbon  modification  present  and  the  temperature,  since  these 
factors  are  determinative  for  the  magnitudes  f  and  rj.  If  we 
consider  now  the  relations  at  constant  temperature  we  see  that 
with  the  same  modification  of  carbon  (constant  £)  P  becomes 
smaller  with  increasing  77. 


176  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  less  noble  the  metal  is,  the  smaller  will  be  the  equilibrium 
pressure  for  the  reduction  of  its  oxide  with  carbon.  It  is  the 
higher  the  more  noble  the  metal.  Now  we  understand  why  the 
nickel  acts  only  as  a  contact  material  toward  the  carbon  monoxide  ; 
why  it  is  not,  as  under  certain  conditions  with  iron,  oxidized  by 
the  decomposing  gas. 

Nickel  is  somewhat  more  noble  than  iron  and  we  may  assume 
that  its  equilibrium  pressure  is  many  atmospheres,  so  that  under 
ordinary  working  conditions  the  decomposition  hyperbola  of 
the  carbon  monoxide  runs  far  beneath  this  pressure.  Iron  is  the 
only  metal  with  which  this  pressure  can  be  conveniently  observed, 
with  the  more  noble  it  is  so  high  that  it  is  difficult  of  measure- 
ment and  with  the  less  noble  it  is  exceedingly  small. 

If  we  consider  only  one  metal  (constant  TJ)  but  different  modi- 
fications of  carbon,  the  equilibrium  pressure  is  the  greatest,  with 
the  form  with  greatest  f.  Accordingly  a  greater  equilibrium 
pressure  corresponds  to  the  equilibrium  with  amorphous  carbon 
than  with  graphite. 

The  magnitudes  of  the  equilibrium  pressures,  for  different 
forms  of  carbon  are,  as  can  be  seen  from  the  equation 


directly  proportional  to  the  values  of  £. 

Both  77  and  £  are  dependent  on  the  temperature  and,  indeed, 
they  both  increase,  as  we  have  seen  earlier,  with  rising  tempera- 
ture. Whether  P  also  increases  with  the  temperature,  cannot 
be  derived  from  the  equation  directly.  It  depends  on  the  ratio 
of  the  temperature  coefficients  of  the  two  equilibrium  constants. 
Experiment  has  shown  that  the  pressure  increases  with  the  tem- 
perature. 

For  the  determination  of  the  equilibrium  pressure,  several 
ways  are  available.  We  can  reach  it  as  we  have  seen  above,  if 
we  allow  carbon  monoxide  at  a  sufficiently  high  original  pressure 
to  react  with  metallic  iron  at  a  given  temperature.  It  is  only 
necessary  to  follow  the  pressure  decrease  and  the  reaction  comes 
to  rest  when  the  equilibrium  pressure  is  reached.  We  arrive  at 
the  same  value  by  gradual  heating  of  the  solid  phases  iron,  carbon 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


177 


and  ferrous  oxide,  in  a  previously  evacuated  tube  connected  with 
a  manometer. 

Investigations  concerning  these  reduction  equilibria  have 
been  carried  out  by  Schenck  and  Heller  *  as  well  as  Schenck, 
Semiller  and  Falcke.f  Their  results  are  collected  in  the  fol- 
lowing table.  Besides  the  pressure  values  the  table  gives  the 
composition  of  the  gas  phase. 


Temperature 
in  Degrees. 

Pressure, 
Mm. 

COMPOSITION  OF  THE 
GAS  PHASE. 

Observers. 

CO, 

Per  Cent. 

CO2, 

Per  Cent. 

455 

27 

510 

43 

538 

81 

552 

130 

53-7 

46.3 

556 

137 

53-4 

46.6 

561 

142 

53-6 

46.4 

Fe,  FeO,  C  am- 
orphous from 
CO 

562 
586 
596 

177 
266 
296 

55-5 

44-5 

Schenck  and 
Semiller 

616 

401 

619 

411 

56.8 

43-2 

629 

469 

643 

561          | 

651 

57i 

57-9 

42.1 

662 

662 

58.4 

41.6 

670 

858 

408 

5-6 

465 

10.4 

Fe,  FeO,  sugar 

560 

161.  7 

Schenck  and 

carbon 

590 

3I4-5 

Heller 

627 

546.8 

649 

750-1 

500 

12.3 

536 

27-3 

Fe,  FeO,  graph-  < 

568 

36.8 
49-2 

Schenck  and 
'     Falcke 

ite 

582 

69-3 

609 

77-5 

660 

129.0 

59-6 

40.4 

Ber.,  38,  2131  (1905). 


tBer.,  40,  1708(1907). 


178 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


As  carbon  modifications,  the  amorphous  carbon  resulting  from 
the  decomposition  of  carbon  monoxide,  sugar  carbon,  and  graph- 
ite are  used. 

It  can  be  seen  from  the  equation,  that  at  the  same  tempera- 
ture, differences  of  pressure  due  to  variation  of  the  carbon  modi- 
fication do  not  also  require  differences  in  gas  compositions. 
These  changes  influence  only  the  values  of  f  but  not  that  of  77. 

Also  the  reduction  of  FesC^  to  FeO  by  means  of  amorphous 
carbon  has  measurable  equilibrium  pressures  which  are  collected 
in  the  following  table: 


Solid  Phases. 

Temperature 
in  Degrees. 

Pressure  in 
Millimeters. 

Observers. 

528 

121 

535 

129 

Fe3O4,  FeO,  C  (amorphous) 

55i 
560 

about  175 
303 

Schenck  and  Falcke 

568 

about  430 

58i 

699 

The  Analysis  of  Mixtures  of  Graphite  and  Amorphous  Carbon. 

We  will  now  put  the  results  of  our  theoretical  and  experi- 
mental investigations  concerning  the  reduction  of  iron  oxide  by 
carbon  to  a  practical  test.  We  will  first  consider  an  analytical 
problem.  The  chemist  has  often  been  given  the  problem  of 
determining  quantitatively  graphite  in  the  presence  of  amorphous 
carbon.  This  problem  has  been  impossible  of  solution  with  the 
previously  available  methods.  Our  intimate  knowledge  of 
reduction  by  carbon  puts  us  in  a  position  to  arrive  at  a  method 
which  will  reach  the  stated  go'al.  It  can  be  directly  deduced  from 
the  diagram,  which  represents  the  equilibrium  between  iron  and 
ferrous  oxide,  as  well  as  the  different  carbon  modifications  with 
the  gaseous  oxides  of  carbon  at  constant  temperature. 

We  show  on  Fig.  99  beside  the  vertical  line  for  the  iron 
equilibrium,  also  the  equilibrium  isotherm  for  amorphous  and 
graphitic  carbon.  We  select  a  suitable  temperature  of  about 
750°.  From  this  diagram  we  arrive  at  the  conditions  under 
which  amorphous  carbon  will  be  oxidized  by  ferrous  oxide  while 
the  graphite  will  remain  unattacked. 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


179 


Our  earlier  considerations  (compare  page  140  and  Fig.  98) 
show  that  a  transformation  of  carbon  into  its  gaseous  oxides  by 
ferrous  oxide  is  only  possible  inside  field  IV  as  in  Fig.  99,  field  IV 
for  amorphous  carbon  is  considerably  greater  than  the  corre- 
sponding one  of  graphite  (curve  G  =  graphite,  A  =  amorphous 
carbon.)  The  remaining  fields  are  not  suitable  for  our  purpose 
since  both  forms  of  carbon  react  with  ferrous  oxide.  The  shaded 


Pmm. 


Isotherms 
for  700° 


III 


0 
100 


20 

80 


30 
70 


50 
50 


60 
40 


900 
800 
700 
600 
500 
400 
300 
200 
100 


80    90   100$  CO 
20    10 


FIG.  99. 


field  (IV)  in  which  only  the  amorphous  carbon  is  burnt  to  car- 
bon monoxide  is  however  suitable. 

In  the  practical  performance,  we  mix  the  sample  of  mixed 
carbon  forms  with  ferrous  oxide  or  more  conveniently  dry  ferrous 
oxalate,  which,  by  heating  to  300°  in  an  indifferent  atmosphere, 
leaves  ferrous  oxide.  If  we  now  heat  this  mixture  in  a  stream 
of  mixed  carbon  monoxide,  and  dioxide,  the  gas  pressure  and 
composition  represents  a  point  in  field  IV,  so  we  have  the  con- 
ditions under  which  the  amorphous  carbon  is  burnt.  If  we 


180  THE   PHYSICAL   CHEMISTRY   OF  THE  METALS 

choose  for  the  performance  a  temperature  between  700°  and  720°, 
which  can  be  held  sufficiently  constant  by  means  of  a  Hergeus 
resistance  furnace,  we  can  work  under  atmospheric  pressure. 
A  convenient  composition  of  the  gas  mixture  is  65  per  cent 
monoxide  and  35  per  cent  dioxide. 

The  Blast-furnace  Process. 

Further,  our  discussion  concerning  reduction  forms  the  basis 
for  the  theory  of  our  most  important  technical  process,  the  iron- 
blast  furnace.  As  we  all  know  the  blast  furnace  consists  of  a 
shaft  of  double  conical  form  in  which  the  iron  ore  to  be  reduced 
is  interlayed  with  carbon  and  slag-forming  material  in  the  upper 
cone.  The  under  bowl  sets  with  its  narrow  part  in  the  "  frame," 
in  which  the  supply  for  the  hot  blast,  the  so-called  "  tuyeres," 
are  built  in. 

The  pit  still  beneath  forms  the  hearth  in  which  the  blast 
furnace  products,  liquid  pig  iron  and  above  this  the  slag  settle, 
the  latter  flows  continuously  through  a  side  opening,  while  the 
metal  is  drawn  from  time  to  time. 

The  process  with  which  we  are  concerned  takes  place  between 
the  tuyeres  and  the  throat.  In  front  of  the  tuyeres  the  carbon 
is  burnt  by  the  oxygen  of  the  air  and  as  a  result  of  the  pre- 
heating of  the  air  there  is  a  very  high  temperature  of  combustion, 
which  is  above  1100°,  the  combustion  product  is  accordingly 
nearly  pure  carbon  monoxide  since  the  gas  mixture,  which  is  in 
equilibrium  with  carbon  at  this  high  temperature  and  the  lowest 
possible  pressure,  of  the  combustion  product  contains  only  traces 
of  dioxide. 

As  at  the  ordinary  pressure  of  750  mm.  the  partial  pressure 
of  the  oxygen  is  150  mm.  and  since  the  two  volumes  result  from 
one  volume  of  oxygen  the  pressure  maximum  is  ^  atmosphere 
(two  volumes  CO— 4  volumes  nitrogen)  i.e.,  250  mm.  The  sum 
of  the  partial  pressures  of  CO2  and  CO  cannot  exceed  this  pres- 
sure and,  as  a  rule,  it  remains  under  this  value. 

The  gases  containing  CO  come  up  from  below  and  encounter 
the  solid  charge  which  melts  in  the  hot  zone  and  is  displaced  by 
fresh  material  from  above.  On  their  way  the  hot  gases  give  up 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC.  181 

their  temperature  and  finally  enter  into  reaction  with  the  heated 
ore  and  carbon.  We  obtain  thus  a  temperature  gradient  inside 
of  the  furnace  from  below  to  above. 

At  the  throat  of  the  furnace  the  gases  still  have  a  temperature 
of  400-500°.  In  this  upper  zone  chemical  reactions  do  not  take 
place  but  the  moisture  which  the  charge  contains  is  vaporized 
and  so  a  drying  of  the  solid  material  results.  With  sinking  into 
the  hot  sphere  the  reduction  begins,  first,  that  of  the  higher 
oxides  to  ferrous  oxide,  then  that  of  ferrous  oxide  .to  metal. 
This  is  cemented  by  a  further  action  of  the  carbon  monoxide 
and  carbon.  Somewhat  below  the  tuyeres  the  temperature 
finally  becomes  so  high  that  both  the  iron  and  the  slag  melt. 
This  liquid  mass  collects  in  the  hearth  and  the  task  of  blast  fur- 
nace, namely  the  preparation  of  pig  iron,  is  finished. 

The  problem  is  to  conduct  the  process  so  that  the  single 
processes  which  go  on  from  oxidized  ore  to  the  iron  carbon  alloys, 
take  place  smoothly  and  without  disturbance.  The  right  tem- 
perature and  the  right  composition  of  the  gas  stream  are  of  the 
greatest  importance  to  reach  this  goal. 

Before  we  go  into  the  possible  disturbances  we  will  turn  to  an 
important  by-product  of  the  blast  furnace,  the  throat  gases. 
Still  at  the  beginning  of  the  ipth  century  the  gases  were  simply 
allowed  to  escape  from  the  throat,  it  was,  however  noticed  that 
they  contained  a  large  amount  of  burnable  substance  which,  if 
allowed  to  go  to  waste,  meant  the  loss  of  large  heat  values.  In 
1836,  Fabre  du  Faure  sought  to  remedy  this  disadvantage  and  by 
the  use  of  the  throat  gas,  for  regenerators,  brought  the  blast  fur- 
nace process  to  a  rational  form.  At  this  time  the  demands  on 
iron  due  to  the  introduction  of  railroads  and  steam  machines  and 
interest  in  cheap  prices  for  the  raw  materials  was  growing  steadily, 
and  caused  everything  necessary  for  economic  production  of  the 
pig  iron  to  be  energetically  considered,  and  occasioned  also  in 
1836  the  Kurfurstliche  Hessische  Bergdirektion  zu  Kassel,  to 
put  to  the  professor  of  chemistry  at  Marburg,  Robert  Bunsen, 
the  problem  of  making  a  close  investigation  of  the  blast-furnace 
process. 

The  title  of  his  communication  on  the  results  of  his  research 


182  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

in  Poggendorffs  Annalen,  "  Concerning  the  gaseous  products  of 
the  blast  furnace  and  its  use  as  burning  material,"  betrayed  at 
once  from  which  side  he  attacked  the  problem.  It  was  on  this 
occasion  that  he  worked  out  his  well-known  methods  of  gas 
analysis  which  he  used  for  the  study  of  the  composition  of  the 
blast-furnace  gases.  He  confirmed  their  high  CO  content  and 
calculated  that  under  certain  conditions  up  to  three-fourths  of 
the  carbon  used  was  lost  if  the  gases  were  allowed  to  escape 
unused,  into  the  atmosphere. 

The  amount  of  combustible  constituents  is  in  fact  large  and 
the  following  small  table  gives  the  average  composition  of  the 
blast-furnace  gases: 

Nitrogen 54~66  per  cent 

Carbon  Dioxide 7~iQ 

Carbon  Monoxide 21-31 

Hydrogen i-  6 

Hydrocarbons 0-6 

The  technic  has  made  use  of  this  knowledge  and  the  gases  are 
led  out  and  used  in  the  rational  way.  In  part,  they  are  burnt  in 
the  Cowper  apparatus  and  so  warm  the  blast  which  the  furnace 
receives  through  the  tuyeres  and  in  part  they  are  used  to  heat 
steam  boilers.  Later  experience  has  taught  that  their  energy 
content  is  best  used,  if  they  are  purified  and  mixed  with  air  in 
gas  engines.  The  useful  effect  is  here  much  greater  than  by  the 
use  in  connection  with  boiler  and  steam  engines.  Blast-furnace 
gas  motors  of  very  large  dimensions  are  now  used.  They  fur- 
nish the  mechanical  energy  for  the  operation  of  steel  and  rolling 
mills,  as  well  as  drive  dynamos  and  furnish  light  and  power  for 
a  wide  circle  of  our  industrial  operations.  The  exploitation  of 
this  original  by-product  has  been  of  the  greatest  technical  im- 
portance. 

Yet  before  we  had  progressed  so  far,  the  question  was  fre- 
quently asked  whether  it  was  possible  to  use  the  strongly  reducing 
gas  for  further  reduction  of  ore.  Large  means  have  been  used 
to  lengthen  the  layer  of  ore  through  which  the  gas  passed.  Blast 
furnaces  have  even  been  built  30  meters  high;  however,  no  matter 
how  high  they  were  the  CO  content  of  the  throat  gases  was  not 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC.  183 

appreciably  reduced.  This  negative  result  of  costly  experi- 
ments cannot  be  wondered  at  by  one  conversant  with  chemical 
equilibrium,  he  knows  that  the  reduction  of  iron  is  a  reversible 
reaction,  he  knows  that  by  the  action  of  FeO  on  CO  a  certain 
amount  of  the  gas  must  remain  over  and  that  the  remainder  does 
not  come  into  consideration  in  the  least  for  the  preparation  of 
iron.  However,  this  scientific  knowledge  is  unfortunately 
younger  than  the  troubles  concerning  the  utilization  of  blast- 
furnace gases. 

The  cooler  is  the  zone  in  which  the  gases  arrive  the  higher  rises 
the  content  of  carbon  dioxide.  This  is  not  alone  due  to  the 
reduction;  the  same  phenomena  would  be  observed  if  the  ore  in 
the  furnace  were  entirely  lacking.  The  influence  of  the  temper- 
ature on  the  composition  of  the  gases  is  due  to  its  effect  on  the 
equilibrium  of  carbon  monoxide  with  carbon.  With  decreasing 
temperature  the  carbon  monoxide  formed  at  the  tuyeres,  breaks 
down  more  and  more  into  dioxide  and  finely  divided  carbon. 
The  presence  of  metallic  iron  greatly  favors  this  reaction.  The 
attainment  of  the  theoretical  equilibrium  is,  however,  only  par- 
tial, due  to  the  great  velocity  of  the  gas  stream.  Therefore,  the 
blast-furnace  gases  are  always  richer  in  carbon  monoxide  than 
corresponds  to  the  equilibrium  ratio  and  the  zone  temperature. 
Since  the  temperature  at  which  reduced  iron  is  in  contact  with 
the  gas,  is  quite  high,  and  the  pressure  of  the  total  equilibrium 
between  metal,  oxide,  carbon  and  the  gases  corresponding  to 
these  temperatures  is  over  |  atmosphere,  we  may,  accordingly, 
designate  the  field  of  the  equilibrium  diagram  that  represents 
the  pressure  and  the  composition  of  the  blast-furnace  gases. 
According  to  the  above  laid-down  rules  only  field  I  comes  into 
consideration  and,  indeed,  only  that  part  in  which  the  pressure 
is  smaller  than  the  pressure  of  the  total  equilibrium.  We  recog- 
nize therefrom  that  the  reduction  in  the  blast  furnace  is  essen- 
tially by  means  of  CO. 

Under  certain  conditions,  for  example,  as  a  result  of  stop- 
pages in  the  blast  supply,  the  region  in  which  the  ore  is  reduced 
to  metal  may  be  cooled  to  a  temperature  of  500°  or  still  lower. 
At  this  temperature  the  pressure  of  the  total  equilibrium  is  less 


184  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

than  the  sum  of  the  partial  pressure  of  monoxide  and  dioxide, 
the  composition  of  the  gases  then  represents  a  point  in  field  II. 
Oxidation  of  the  metal  takes  place  and  simultaneous  precipitation 
of  finely  divided  carbon,  which  stops  the  furnace.  The  approach 
of  danger  may  be  learned  from  the  increase  of  C02  in  the  throat 
gases.  The  lowest  temperature  at  which  reduced  metal  may  be  in 
contact  with  the  furnace  gases,  without  reoxidation  of  iron  taking 
place,  is  that  at  which  the  pressure  of  the  total  equilibrium  is 
equal  to  the  sum  of  the  partial  pressure  of  the  carbon  monoxide 
and  dioxide  in  the  gases.  If  we  take  this  sum  as  200  mm.,  the 
temperature  above  which  no  difficulty  will  ensue  is  570°.  Be- 
side the  reduction,  there  takes  place  in  the  furnace,  the  cement- 
ing of  the  iron,  which  is  a  very  important  process  for  the  smooth 
operation  of  the  blast  furnace.  As  we  have  seen  in  an  earlier 
lecture  the  melting  point  of  the  iron-cementite  is  very  much  lower 
than  that  of  pure  iron.  This  fact  has  the  important  practical 
consequence,  that  one  can  operate  a  furnace,  in  which  pig  iron 
is  to  be  melted,  at  a  much  lower  temperature  than  one  in  which 
pure  iron  is  to  be  melted.  This  temperature  difference  is  nearly 
400°,  a  condition  which  is  very  essential  for  the  permanence  of 
the  furnace  material. 

Concerning  the  conditions  for  the  cementing  of  iron  by  car- 
bon monoxide  we  will  supplement  the  experiments  by  means  of 
theoretical  calculation.  We  are  limited  for  experimental  mate- 
rial to  an  investigation  of  the  question  at  temperatures  up  to 
700°,  below  this  the  formation  of  solid  solutions  of  iron  and  car- 
bide does  not  occur,  the  results  of  these  experiments  give  us  only 
a  general  schematic  picture  of  the  run  of  cementation  at  higher 
temperatures  and  with  solid  solution  formation. 

In  laboratory  investigations  concerning  the  action  of  CO  on 
metallic  iron,  it  has  frequently  been  observed  that  if  the  amount 
of  carbon  monoxide  present  is  large  compared  to  the  amount  of 
iron,  that  there  are  entirely  different  and  much  lower  equilib- 
rium pressures  than  under  the  same  conditions  of  temperature 
and  original  pressure  with  relatively  smaller  amounts  of  gas  and 
larger  amounts  of  metal.  The  former  pressure  value  is  near  that 
which  represents  the  equilibrium  between  iron,  FeO,  graphite  and 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


185 


the  gases  and  its  origination  is  explained  by  the  transition  of 
labile  amorphous  carbon  into  the  stable  graphite. 

On  the  basis  of  our  earlier  experiences  we  have  a  means  of 
testing  this  explanation  for  its  correctness.  If  the  difference  of 
the  twQ  pressures  is  conditional  on  the  carbon  modifications, 
there  must  be  at  constant  temperature,  the  same  ratio  of  carbon 
monoxide  and  carbon  dioxide,  the  monoxide  content  of  gas  must 
vary  accordingly  between  53  per  cent  and  60  per  cent  (see  table 
p.  143).  To  our  astonishment  the  content  proves  to  be  much 
higher,  between  85  and  90  per  cent.  The  difference  of  the  two 
equilibrium  systems  can  not  accordingly  be  conditioned  by  the 
difference  in  the  carbon  modifications. 

Another  possible  explanation  is  that,  in  the  place  of  FeO, 
another  more  difficultly  reducible  oxide  has  been  formed,  but 
there  is  no  other  evidence  of  this.  The  only  logical  assumption 
is  that  a  change  takes  place  in  the  iron  phase.  Since  by  the  ac- 
tion of  carbon  monoxide  on  metallic  iron,  there  takes  place 
first,  a  transformation  of  the  upper  surface  into  cementite  (FesC), 
according  to  the  equation 


and  this  carbide  is  oxidized  by  further  decomposition  of  the  CO, 
as  we  have  earlier  seen  with  the  metal,  till  finally  there  results  a 
total  equilibrium  between  cementite,  ferrous  oxide,  amorphous 
carbon  and  the  gas  mixture.  The  observation  data  follow: 


COMPOSITION  OF  GAS. 

Tempera- 
ture, 
Degrees. 

Pressure, 
Millimeters. 

Observers. 

CO, 

CO2, 

Per  Cent. 

Per  Cent. 

468 

10.3 

O.O 

O.O 

540 

30.0 

600 

6<?  o 

364 
672 

U0  .  w 

82.9 
131 

Schenck,  Heller,  Semiller,  and 
Falcke. 

86.0 

14.0 

69I 

195 

722 

298 

87.5 

12-5 

•'-.      * 

734 

34i 

88.0 

12.  0 

735 

344 

88.3 

ii.  7 

, 

774 

562 

89-5 

10.5 

\  Presence  of  solid  solutions. 

779 

657 

J 

186 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


The  iron  carbide  is  accordingly  considerably  more  easily 
oxidized  than  the  metal. 

If  the  dependence  of  the  pressure  on  the  temperature  is 
shown  graphically,  a  curve  is  obtained  exactly  as  with  the 
reduction  of  ferrous  oxide  by  the  different  modifications  of  car- 
bon. In  order  to  represent  the  comparative  relations  of  this 
uni variant  system,  the  composition  of  the  gas  phase  must  be 
considered,  that  is,  a  space  model  must  be  used  with  the  coor- 
dinates T,  temperature,  P,  pressure  and  x,  composition.  In 


800 
700 
600 
500 
400 
300 
200 
100 


C0+C02  Pressure 
m  m. 

Ill 


400°  450°  500°  550°  600°  650°  700°  750°  800°  Temp, 
FlG.  100. 


this  the  equilibria  are  represented  by  space  curves.  If  we  pro- 
ject these  space  curves  on  the  three  coordinate  planes,  we  obtain 
Figs.  100  and  101. 

From  these  observation  data  the  conditions  can  be  deduced, 
under  which  iron  carbide  is  formed  from  metallic  iron.  We  have 
even  determined  a  series  of  space  curves  which  represent  the 
many  equilibria  of  the  four-phase  system.  If  we  consider  a 
single  isotherm  in  this  space  diagram,  the  intersections  of  the 
single  space  curves  are  marked  as  points. 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


187 


Concerning  the  4-phase  systems  (Fe,  FeO,  C  (amorphous)  CO, 
C02)  (Fe,  FeO  graphite,  CO,  CO2)  and  Fe304,  FeO,  C  (amorph- 
ous) CO,  C02)  we  have  earlier  seen  that  these  points  can  be 
conceived  of  as  intersections  of  the  curves  of  bivariant  equi- 
librium systems.  The  same  holds  naturally  also  for  the  new 
systems  (Fe3C,  FeO,  C  (amorphous)  CO,  C02).  We  think  of 
these  four  phases  as  conditioned  on  the  simultaneous  existence 
of  the  equilibria 

2CO<=»C+C02, 


3FeO+5CO  <=>  Fe3C+4C02. 


°S      °8  "   bS      °§ 


-i 1 — 

Fe,C,  FeO,  00,002 


Fe,  Fe  0,  CO, 


§       8 
FIG.  101. 


§m  m, 


The  conditions  for  the  first 


We  already  know,  for  the  second  mass  law  gives  the  constant 

v5 

rP. 


—  -rH" 


The  graphic  representation  for  the  first  equation  is  the  cubic 
hyperbola  with  which  we  are  familiar,  the  second  equation  is 
represented  by  a  similar  curve  of  a  higher  order,  which,  beside 
the  starting  point  (P  =  0,x  =  i)  cuts  the  carbon  curve  in  still  a 
second  point. 


188  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  coordinates  of  this  intersection  can  be  deduced,  in  a  very 
simple  way,  from  the  two  equations  which  must  be  simulta- 
neously fulfilled.  The  division  of  the  second  by  the  first  gives 


From  this  and  from  the  first  it  follows: 


From  the  isotherm,  in  which  we  represent,  beside  the  two 
above  treated  curves,  still  the  vertical  straight  lines  for  the 
equilibrium: 

FeO+CO^Fe+C02, 

with  the  equation, 


we  see  that  the  new  curve  (see  Fig.  102)  in  its  lower  part  is 
strongly  curved  and  above  it  is  a  straight  line  nearly  normal  to 
the  x  axis.  Its  tendency  toward  the  left  is  only  small,  never- 
less  it  causes  an  intersection  of  this  curve  of  higher  order  with  the 
equilibrium  line  for  the  system  (Fe,  FeO,  CO,  C02)  at  high  values 
of  P.  (Compare  here  Fig.  103).  At  the  intersection  the  two 
equations 

—  =  r?    and        x    v4-P  =  fl, 

I  ~~~X  \L      X) 

are  simultaneously  fulfilled;  from  these  the  coordinates  of  the 
intersection  follow: 

x—— — ,    and    P  = 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


189 


This  point  represents  a  new  univariant  equilibrium  not  pre- 
viously treated  and  not  directly  observable,  between  the  four 
phases,  Fe,  FeO,  FesC,  and  gas.  We  may  represent  it  by  the 
symbol 

FeO+Fe3C  <=>  4Fe+CO. 

The    corresponding    equilibrium    pressure    cannot   be   directly 
measured,  but  we  are  able  to  calculate  it  from  the  equilibrium 


800 
700 
600 
500 
400 
300 
200 
100 


Pressure 
m.m. 


Isothermes  for  650" 


0        10       20       30       40       5,0       60       70       80       90      100 

$CO 

FIG.  102. 

pressure  of  the  systems  we  can  observe.     The  calculation  gives 
for  the  temperatures  650  and  700°,  the  values 

P<>5Q°  =  5I-92  atm.;  P700°  =  166.3  atm. 

The  composition  of  the  gases  is  naturally  the  same  as  in  the 
reduction  of  FeO,  by  carbon  monoxide,  forming  metal  and  car- 
bon dioxide  as  the  calculated  points  already  belong  to  the 
equilibrium  lines  for  this  reaction  system.  Accordingly  X65Qo  = 
0.58  and  X70QO  =0.60. 


190 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


The  calculation  of  this  not  directly  observable  equilibrium 
is  not  as  it  might  appear  idle  play.     The  coordinates  found  in 


CO  i.e. 


FIG.  103. 

this  way  give  us  a  knowledge  of  a  magnitude  that  is  of  great 
practical  importance.     It  makes  it  possible  to  give  the  condi- 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC.  191 

tions  under  which  metallic  iron  is  cemented  by  carbon 
monoxide. 

The  above-determined  equilibrium  point  is  not  only  the  inter- 
section for  the  curves  of  the  bi  variant  system  (Fe,  FeO,  CO,  C02) 
and  (FeaC,  FeO,  CO,  CQ2).  Since  in  it  three  solid  phases  are  in 
equilibrium  with  the  gas,  the  equilibrium  curve  for  the  third 
possible  combination  (FesC,  Fe,  CO,  CO2)  must  also  pass 
through  it.  The  equilibrium  3Fe+2CO  <=±  Fe3C  +  C02  also 
exists  at  the  calculated  point.  This  equilibrium  is  also  bivariant 
since  it  consists  of  three  components  (C,  Fe,  O)  simultaneously 
present  as  three  phases.  . 

The  equilibrium  curve  has,  exactly  as  for  the  coexistence  of 
carbon  in  contact  with  its  gaseous  oxide,  the  equation 


it  is  accordingly  a  cubic  hyperbola.  The  constant  ju  is  connected 
with  the  two  constants  77  and  #  through  a  simple  relation.  For 
the  two  intersection  points,  we  have 

Pi~\"n        j     r>     Oi~l~*? 
=  )u ,     ana    *?.**& . 

From  this  follows: 

For  650°  #=83,240  and  17= 1.381,  for  700°  5  =  384,000  and  77  =  1.500 

hence 

M650°  =  13600;    M700°  =  113,800. 

From  these  values  and  the  equation 

x" 


I—X 


•P, 


the  composition  of  the  gases  can  be  calculated  which  are  in 
equilibrium  with  iron  and  cementite  at  a  determined  pressure; 
they  are 

=  °-96,  and  #700  =  0.99. 


192  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

At  the  temperatures  650°  and  700°  accordingly  a  transforma- 
tion of  iron  into  cementite  is  only  possible  with  such  mixtures  of 
CO  and  C02  which  contain  more  than  96  and  99  per  cent  CO, 
respectively.  The  position  of  the  cementation  curve  is  shown 
in  the  isotherm  for  650°  (see  Fig.  102)  and  one  can  get  from  this 
the  necessary  CO  concentration  for  carbide  formation  at  other 
pressures.  From  the  equilibrium  diagram  for  650°  (Fig.  102) 
we  know  further  that  ferrous  oxide  goes  into  cementite  easier 
than  the  metal;  86  per  cent  being  sufficient  for  that  reaction. 

In  the  previously  considered  cases  pure  cementite  always 
results.  One  dare  not  draw  the  conclusion  from  these  investi- 
gations that  the  conditions  for  cementation  always  require  so 
high  a  content  of  monoxide.  If  the  temperature  allows  of  the 
formation  of  solid  solutions  between  iron  and  carbon  the  relations 
are  somewhat  different.  The  influence  of  the  solid  solution  for- 
mation on  the  CO  concentration  necessary  for  the  cementation 
can  also  be  seen  from  the  schematic  diagram  (see  Fig.  103). 

In  this  diagram  we  recognize  again  our  four  curves  for  the 
bivariant  equilibria: 

(A)  2CO  ^C  +  C02 

(B)  FeO+CO     <=»Fe+C02 

(C)  3FeO  +  5CO  <=>  Fe3C+4C02 
CD)  3Fe+2CO    <=±Fe3C+C02. 


including  their  intersections.  If  some  of  the  iron  dissolves  car- 
bide so  must  the  lines  representing  the  equilibrium  with  metallic 
phases,  the  lines  b  and  the  curve  d  be  displaced.  The  sense  of 
this  displacement  is  at  once  clear.  Through  the  presence  of  the 
easily  oxidized  carbide,  the  solid  solution,  compared  with  the 
pure  iron,  is  also  increased  in  oxidizability  ;  it  requires  for  trans- 
formation into  FeO  even  smaller  C02  concentrations  than  the 
pure  metal;  that  is  the  line  b  is  displaced  to  the  right.  The  result 
of  this  changed  position  is  a  sinking  of  the  pressure  value  for  the 
four-phase  system.  The  pressure  of  the  system  (solid  solu- 
tion FeO,  Fe3C,  CO,  CO2)  is  smaller  than  that  for  the  system 
(ferrite,  FeO,  Fe3C,  CO,  C02).  Since  the  cementation  curve 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC.  193 

for  the  solid  solution  must  go  through  this  point,  it  can  be  seen 
directly  from  the  figure,  that  the  cementation  of  solid  solutions 
can  take  place  with  lower  monoxide  concentrations  than  that 
of  pure  iron.  The  cementation  curve  is  pushed  toward  the  left 
(the  curve  for  the  solid  solution  is  dotted). 

Since  the  curve  c  is  very  steep  relatively  small  displacements 
of  x,  respectively  to  t\  represent  large  depressions  of  the  inter- 
section -4.  The  ordinate  equation 


also  holds  for  this  point.    &  retains  its  value  since,  in  the  equi- 
librium c  no  solid  solution  phase  appears. 

How  large  this  influence  is,  a  rough  calculation  will  show. 
We  will  assume  that  at  a  temperature  of  700°  solid  solution  for- 
mation already  takes  place  and  that  through  this  condition  the 
equilibrium  value  X  for  the  oxidation  of  this  solid  solution  is 
raised  to  0.62  (compared  to  0.60  for  pure  iron).  Thereby  t\  in- 
creased from  1.50-1.63.  The  value  of  #  is  at  this  temperature 
384,000.  From  the  above  given  equation  we  calculate  for  the 
equilibrium  pressure  of  the  solid  solution  with  the  simultaneous 
presence  of  cementite  and  FeO,  the  value  115.4  as  compared  to 
166.3  atm.  if  we  replace  the  solid  solution  with  pure  iron.  The 
depression  is  accordingly  51  atm.  for  the  small  concentration 
increase  of  CO  (around  2  per  cent).  That  the  constant  n  of  the 
cementation  equilibrium  (FeaC,  Fe,  gas)  changes  with  a  dis- 
placement of  77  is  shown  by  the  equation: 


Accordingly  this  change  is  relatively  large  for  small  deviations  of 
the  variable  77,  since  M  is  approximately  proportional  to  the  third 
power  of  77.  It  decreases  with  the  above  given  change  of  77  from 
113,800  to  88,650. 

Beside  the  intersection  A,  the  intersection  of  curves  a  and 
b  is  depressed  by  solid  solution  formation.  Since  the  curve  a 
is  flatter  than  the  curve  d,  the  influence  is  not  so  great  as  with  a, 


194 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


but  it  is  sufficiently  large,  that  the  pressure  value,  which  is  above 
atmospheric  pressure  for  pure  iron,  is  depressed  below  this  value 
so  that  the  equilibrium  of  the  solid  solution  becomes  conveniently 

G 


CO  i.e.-    x 


FIG.  104. 

measurable.  And  this  is  not  without  practical  importance,  since 
we  may  orient  ourselves  experimentally  concerning  the  depend- 
ence of  the  constant  rj  on  the  carbide  content  of  the  solid  solution 
and  from  the  determined  relations  follow  the  influence  of  the  solid 


DECOMPOSITION  OF  CARBON  MONOXIDE,   ETC.  195 

solution  formation  on  the  cementation  process  mathematically. 
Nothing  further  being  necessary,  than,  after  the  occurrence  of 
equilibrium  between  solid  solution,  ferrous  oxide,  carbon  and 
the  gas  atmosphere,  to  determine  the  composition  and  pressure 
of  the  latter  and  to  analyze  the  solid  solution. 

All  these  experimental  data  still  fail  to  place  us  in  a  position 
to  predict  the  direction  of  the  displacement  of  the  equilibrium 
and  the  reaction  fields  under  the  complex  conditions  for  the 
formation  of  solid  solutions  such  as  can  occur  at  high  tempera- 
tures. 

After  this  digression  we  return  to  the  simple  relation  such 
as  we  have  seen  holds  under  700°,  we  draw  again  a  schematic 
diagram  of  the  equilibrium  curves  and  their  intersections  at  con- 
stant temperature,  considering,  however,  also  the  ferrous  oxide 
(see  Fig.  104). 

We  have,  accordingly  besides  the  representations  of  the 
equilibria 


FeO+CO^»Fe+C02 
3FeO+5CO  <=>  Fe3C+4C02 


also  that  for 

Fe304+CO  <=±  3FeO+CO2 

These  five  curves  divide  our  diagram,  the  X,  P  plane  into  twelve 
fields,  in  which  the  following  reactions  go  on: 

Field  A 


2.  3 

3.  FeO+CO  =  Fe+C02 

4.  Fe304+CO  =  3FeO+C 

5.  2CO  =C+C02 


196  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

Field  Bl  Field  B2 


2. 
3. 
4. 


Fe+CO  =  Fe+CO2 


=  C+C02 


Field 


FeO+CO  =  Fe+C02        3. 

=  C+C02         5.       C+C02 


Field 


2. 
3. 
4. 
5. 


2.  Fe3C+4C02=3FeO+5CO 

3.  Fe+CO2  =  FeO+CO 

4. 
5. 

Field 


2.  e 

3.  Fe+C02  =  FeO+CO 

4. 
5. 


2. 


2. 

3- 
4- 
5- 


2. 


as  n 


Field  C2 


as  in  Ci 


Field  D2 


i. 

2. 

3- 
4- 
5. 


i. 

2. 

3- 
4- 


as  n 


Field  E2 


as  n 


5.       C+C02  = 


Field  F 


3.  Fe+C02  =  FeO+CO 

4.  Fe304+CO-3FeO+C02 
5. 


DECOMPOSITION  OF  CARBON  MONOXIDE,   ETC.  197 

Field  G 


2. 

3.  Fe+C02  =  FeO+CO 

4.  Fe304+CO  =  3FeO+C02 

5.  2CO  =  C+C02 

Field  H 

1.  3Fe+2CO=Fe3C+C02 

2.  Fe3C+5C02  =  3FeO+4C02 

3.  Fe+C02  =  FeO+CO 

4.  3FeO+C02  =  Fe304+CO 

5.  2CO  =  C+C02 

As  end  products  of  the  reaction  also  occur:  Cementite, 
together  with  carbon  in  fields  A  and  F,  metallic  iron  with  carbon 
in  fields  BI  and  Ci,  without  simultaneous  precipitation  of  carbon 
in  I$2  and  C2. 

Ferrous  oxide  with  carbon  in  fields  D\  and  G. 

Ferrous  oxide  without  carbon  in  field  D2. 

Ferrosoferric  oxide  with  carbon  in  fields  E\  and  H. 

Ferrosoferric  oxide  without  carbon  in  field  £2. 

We  can  accordingly,  at  the  same  temperature  obtain  any  of 
the  four  solid  substances  as  the  stable  phase  by  simple  variations 
of  pressure  and  composition  of  the  gas  phase. 

By  raising  the  temperature  all  fields  suffer  deformation, 
all  curve  constants  increase,  as  a  result,  the  curvature  changes 
and  displacements  occur  toward  the  right.  If  we  think  now 
of  all  isotherms  as  vertical  to  a  temperature  axis  of  a  space 
model  with  the  coordinate  axes,  T,  P,  and  X,  so  we  have  a 
geometrical  representation  of  the  whole  theory  of  the  blast 
furnace. 

We  will  now  study  one  important  conclusion  which  our 
diagram  brings  out.  It  so  happens  that  the  cementite  —  as 
a  phase  —  in  opposition  to  the  other  solid  products  is  only  stable 
in  such  fields  as  the  CO  is  labile  and  is  subject  to  decomposition 


198  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

into  C02+C.  From  this  it  arises  that  the  cementite  is  not 
formed  from  carbon  and  metal,  that  the  only  cementing  agent 
by  which  one  can  reach  a  direct  formation  of  carbide  is  CO. 
It  can,  however,  be  formed  by  the  action  of  carbon  on  ferrous 
oxide  (in  B).  It  is,  however,  labile  here. 

However,  the  solid  carbon  can  also  act  as  cementing  agent 
if  it  is  not  in  the  formation  of  pure  cementite  as  a  separate 
phase,  but  in  the  formation  of  a  solid  solution  of  this  substance 
with  metallic  iron.  Suppose  that  the  prevailing  temperature 
for  the  formation  of  solid  solution  falls  inside  of  field  €2, 
the  field  in  which  at  temperature  less  than  770°,  metallic  iron 
is  the  only  stable  solid  phase.  At  this  temperature  reaction  i 
in  the  above  given  table  is  no  longer  unidirectional,  but  is 
reversible  for  a  large  interval  of  X.  We  must  replace  it  with 
the  equation 


in  which  the  carbide  belongs  to  the  same  phase  as  the  metal. 

If  we  remember  the  Mass  Law,  we  see  that  the  composition 
of  the  solid  solution  is  dependent  on  the  pressure  and  com- 
position of  the  gas  atmosphere. 

That  is  also  the  case  for  the  reaction. 

FeO+CO  <=±  Fe  (Fe3C  containing)  +C02. 

The  fields  F  and  G  which  on  account  of  the  high  pressure 
values  are  not  observable  with  iron  deserve  a  few  further  words. 
The  mutual  relations  of  metal,  carbide  and  oxide  in  the  presence 
of  gas  can  be  made  clear  with  other  metals,  especially  with 
manganese. 

Practical  experience  has  taught  that  by  the  reduction  of 
manganese  oxide  with  carbon  and  carbon  monoxide  a  carbon- 
free  metal  never  results  but  always  a  carbide.  We  might  draw 
the  conclusion  from  this  that  the  oxide  is  more  easily  reduced 
to  the  carbide  than  to  the  metal,  which  would  be  the  case  at 
point  A  if  the  CO  concentration  necessary  for  cementation  were 
smaller  than  that  required  for  reduction. 


DECOMPOSITION  OF  CARBON  MONOXIDE,   ETC. 


199 


The  equilibrium  pressure  occurring  with  manganese  is  very 
small.  Carbon  monoxide  is,  up  to  over  1000°,  easily  and  com- 
pletely absorbed  by  metallic  manganese  with  strong  heat  evo- 
lution. At  a  temperature  of  1200°  the  equilibrium  pressure 
reaches  a  measurable  value.  This  relation  can  be  easily  under- 
stood from  our  earlier  theoretical  considerations,  since  with 
manganese  we  have  a  metal  which  is  much  less  noble  than 
iron  and  as  a  consequence  possesses  a  larger  value  for  the  reduc- 
tion constant  rj.  What  holds  for  rj  holds  also  for  $,  the  constant 
which  gives  the  conditions  for  the  reduction  of  the  oxide  to  the 
carbide.  The  result  of  increase  of  ??  and  $  is  the  strong  decrease 


800- 
700- 
600- 
500- 
400- 
300- 
200- 
100- 


m  m. 


Temp. 


600°  700°  800°  900°  1000°  1100° 

FIG.  105. 

of  the  equilibrium  pressure  for  the  systems.  Metal  oxide, 
carbon  (amorphous)  gas,  and  carbide  oxide,  carbon  (amor- 
phous) gas. 

This  influence  makes  itself  felt  in  the  action  of  CO  on  solid 
solutions  of  iron  and  manganese  as  well  as  those  of  iron  and 
manganese  cementite.  Small  manganese  contents  cause,  as 
the  observations  show,  great  decrease  in  the  equilibrium  pressure. 
In  the  following  table  the  results  of  one  experiment  are  given, 
which  has  been  taken  from  the  observations  of  Schenck  and 
Semiller  on  the  action  of  CO  on  manganese  containing  pig  iron. 


200 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


From  the  numerical  values  and  still  better  from  the  graphical 
representation  (Fig.  105)  the  influence  of  the  manganese  content 
on  the  equilibrium  pressure  for  the  presence  of  the  solid  phases, 
cementite,  oxide,  amorphous  carbide  can  be  seen.  For  com- 
parison the  values  for  manganese  free  iron  cementite  are  given. 

COMPOSITION  OF  MANGANESE   CONTAINING   PIG   IRON 


Preparation. 

Mn,  Per  Cent. 

Fe,  Per  Cent. 

C,  Per  Cent. 

I 

o-9S 

96.6 

2.IS 

II 

4.01 

93-o 

2.99 

III 

6.38 

93-2 

1.32 

Cementite.  Manganese 
Free 

PREPARATION  I. 

PREPARATION  II. 

PREPARATION  III. 

Temp,  in 
Degrees. 

Pressure, 
Mm. 

Temp,  in 
Deg. 

Pressure. 
Mm. 

Temp., 
Deg. 

Pressure, 
Mm. 

Temp., 
Deg., 

Pressure, 
Mm. 

634 

83 

629 

28 

672 

131 

679 

41 

69I 

195 

* 

722 

298 

734 

341 

730 

86 

774 

562 

751 

IOI 

779 

657 

820 

180 

849 

217 

853 

17 

880 

349 

900 

396 

911 

28 

943 

IS 

0^8 

617 

0^6 

20 

VOU 

959 

802 

965 

46 

vo 

981 

20 

IOIO 

74 

1031 

34 

1088 

162 

1086 

62 

1093 

68 

1103 

170 

IIIO 

96 

If  we  also  distort  the  results  by  the  formation  of  ferrite- 
cementite  solid  solution,  we  obtain  a  picture,  at  least  qualitative, 
of  the  influence  on  the  equilibrium  of  the  presence  of  man- 
ganese. 

According  to  our  previous  consideration  we  would  expect 
that  the  reduction  of  the  metallic  oxide  by  carbon  and  CO 
would  be  easier  the  more  noble  the  metal  and  coordinately  the 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC.  201 

greater  the  field  B2  of  our  diagram.  We  should  encounter 
accordingly  very  little  danger  of  falling  into  a  field  where  the 
C02  concentration  is  so  high  that  the  metal  present  is  oxidized. 

The  experience  of  practice  and  the  laboratory  show  us  now 
that  one  is  bitterly  deceived  in  these  expectations.  Nickel 
oxide,  as  well  as  tin  oxide,  are  vainly  subjected  to  conditions, 
which  with  iron  oxide  would  produce  the  metal  without  dif- 
ficulty. To  obtain  an  energetic  reaction  much  higher  temper- 
atures must  be  used  than  are  necessary  according  to  the  posi- 
tion of  the  equilibrium  pressure,  which  with  noble  metals  is 
already  many  atmospheres  at  relatively  low  temperatures. 

We  have  heretofore  left  entirely  out  of  consideration  a 
factor  which  is  of  essential  meaning  for  an  energetic  reaction, 
namely :  the  reaction  velocity,  the  velocity  with  which  a  chemical 
process  approaches  the  equilibrium.  Even  with  the  oxides  of 
noble  metals,  the  reduction  with  carbon  is  only  feasible  if  the 
reaction  velocity  is  not  too  small. 

The  resistance,  which  is  opposed  to  an  energetic  completion 
of  the  reaction  is  in  many  cases  due  to  the  physical  condition 
of  the  oxide.  With  one  and  the  same  substance,  we  can  meet 
different  degrees  of  reaction  energy,  if  it  has  received  different 
previous  treatment.  Strongly  ignited  sintered  masses  are  much 
more  inert  than  finely  pulverized  porous  preparations.  The 
surface  is  the  essential  factor  on  which  the  reaction  velocity 
depends. 

A  second  factor  is  the  temperature  and  indeed  the  velocity 
increases  with  the  temperature  under  all  conditions.  Of  this 
fact  use  has  long  been  made  in  practice.  The  reduction  of 
NiO,  which  at  600°  goes  only  very  slowly,  is  conveniently 
carried  on  100°  higher  provided  the  oxide  and  the  reducing 
agent  are  intimately  mixed.  There  must  be  considered  for 
the  reduction  of  oxides,  beside  the  equilibrium  ratio  also  suf- 
ficiently great  reaction  velocity.  In  many  cases  it  is  easier 
to  obtain  a  base  metal  from  its  oxide  than  a  noble  one,  if  the 
latter  oxide  shows  inertness  toward  the  reducing  agent. 


202  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

The  Mond  Nickel  Process. 

We  dare  not  close  our  considerations  of  the  action  of  CO  on 
the  oxides  and  metals  without  mentioning  a  remarkable  reaction, 
which  is  of  interest  to  the  metallurgist  and  has  been  made  by 
Mond  to  play  an  important  role  in  the  production  of  nickel. 

Iron  and  nickel,  have,  at  temperatures  slightly  removed 
from  room  temperature,  the  peculiarity  of  adding  CO  and  form- 
ing with  it  the  remarkable  compounds — Fe  (CO)?,  Fe(CO)5, 
Ni(CO)4. 

The  most  important  of  these  is  nickel  carbonyl  Ni(CO)4. 
Concerning  its  formation  and  existence  conditions,  Mittach  * 
has  made  thorough  investigations.  Nickel-carbonyl  is  a  water 
clear,  very  volatile  liquid  which  boils  at  40°  and  at  higher 
temperatures  decomposes  easily  into  the  components  from 
which  it  was  formed.  The  ease  of  formation  on  the  one  hand 
and  the  decomposition  on  the  other  can  be  demonstrated  by  a 
simple  experiment.  If  we  fill  a  glass  tube,  drawn  to  a  point  on 
both  ends,  with  the  vapor  of  nickel-carbonyl  and  place  it, 
after  the  ends  have  been  sealed,  in  a  boiling  water-bath,  after 
a  time  we  observe  the  precipitation  of  a  nickel  mirror.  If 
then  the  tube  be  taken  out  and  allowed  to  lie  a  few  days  at 
room  temperature,  the  mirror  disappears  again,  with  re-for- 
mation of  nickel-carbonyl  vapor.  We  are  accordingly  dealing 
with  a  reversible  reaction,  which  can  be  expressed  by  the  symbol 

Ni-h4CO<=±Ni(CO)4. 

As  previously,  we  can  deduce  the  equilibrium  conditions  from 
the  mass  law.     They  are 


C  co ' 


K. 


We  have,  so  long  as  the  nickel-carbonyl  is  gaseous,  a  two- 
component  system  with  two  phases;  it  is  accordingly  bivariant 

*  Z.  Physik.  Chem.,  40,  i  (1902). 


DECOMPOSITION  OF  CARBON  MONOXIDE,  ETC. 


203 


and  dependent  on  the  pressure  and  temperature.    The  equation 
of  the  isotherm  is  as  one  can  easily  derive. 


i—  x 


which  gives  us  the  relation  between  the  pressure  and  the  com- 
position of  the  gas  phase.  The  higher  the  pressure  the  greater 
the  amount  of  the  nickel-carbonyl  vapor  in  it. 


Temperature  in 
Degrees. 

logK. 

Temperature  in 
Degrees. 

logK. 

"•3 

2.719 

62.0 

5-575 

16.1 

3-04 

70.4 

5-948 

29.8 

3.812 

8o.O 

6-443 

35-9 

4.199 

90.0 

6.859 

50.2 

4.925 

99-3 

7.178 

It  is  sufficient  to  have  the  values  for  K  or  its  logarithm 
for  a  series  of  temperatures    between   10  and   100°.     From 


10°  20°   30°   40°   50°   60°   70' 
FIG.  106. 


90°  100° 


these  we  can  calculate  the  composition  of  the  gas  for  every 
other  temperature.  For  an  illustration  of  the  displacement  of 
the  equilibrium  with  the  temperature,  see  Fig.  106.  It  shows 
that  as  the  amount  of  CO  in  the  gas  mixture  increases  with 
rising  temperature,  that  of  nickel-carbonyl  decreases. 


204  THE  PHYSICAL   CHEMISTRY  OF  THE  METALS 

The  constants  vary  somewhat  with  the  condition  of  the 
metallic  nickel  present,  and  the  value  for  the  finely  divided 
metal  is  higher  than  for  the  compact,  an  analogy  to  the  fact 
that  small  crystals  show  a  greater  solubility  than  large. 

The  velocity  of  the  formation  is  influenced  by  a  large  num- 
ber of  factors.  That  increasing  pressure,  that  is,  raising  the  CO 
concentration,  is  favorable  to  the  reaction  is  a  result  of  the 
mass  law  and  that  fine  division  of  the  nickel  also  aids  the 
reaction  can  be  understood  from  our  earlier  experience.  It 
is  remarkable,  however,  that  traces  of  foreign  substances,  for 
example  of  oxygen,  cause  very  great  variations  from  the  nornal 
run.  They  work  directly  as  "  poison "  for  the  nickel  and 
destroy  its  combining  ability.  It  would  be  going  too  far  here 
to  follow  this  influence  further. 

Our  problem,  the  action  of  carbon  and  carbon-monoxide 
on  the  ores  and  metals,  and  the  study  of  the  occurring  equilib- 
brium  phenomena  is  now  disposed  of,  and  we  now  understand 
the  factors  on  which  the  oxidation  reduction  and  cementation 
depend.  We  will  now  turn  to  another  field,  application  of 
theory,  to  the  chemical  processes  which  are  important  in  the 
smelting  of  sulfide  ores. 


CHAPTER  VI 
THE  REACTIONS   OF  SULFIDES 

WHILE  for  the  preparation  of  iron  only  oxide  ores  come 
into  consideration,  the  principal  amount  of  the  other  prac- 
tically important  metals  are  obtained  from  the  sulfides.  This 
holds  especially  for  copper,  lead,  zinc,  and  mercury. 

A  thorough  knowledge  of  the  relations  of  the  metallic  sul- 
fides to  the  metals  on  the  one  hand,  and  to  the  oxides  on  the 
other,  is  therefore  essential  if  we  would  specify  the  conditions 
under  which  the  reactions,  which  are  used  in  practice  for  the 
transformation  of  sulfides  to  the  metal,  run  in  the  desired 
direction.  It  is  a  well-known  fact  that  at  high  temperatures 
the  sulfides  undergo  a  dissociation  into  their  elements  or  into 
sulfur  and  sulfur-poorer  sulfides.  From  some,  for  example, 
the  iron  sulfide  FeSs,  the  sulfur  can  be  directly  distilled.  What 
now  are  the  kinds  of  dissociation  phenomena  concerned,  and 
are  the  relations  exactly  as  with  the  dissociation  of  the  oxides. 

The  reaction 

MeS  <±  Me+S, 

is  characterized  by  a  definite  dissociation  tension,  dependent 
on  the  temperature  which  with  the  above-mentioned  iron 
sulfide  amounts  to  over  one  atmosphere  at  the  distillation 
temperature. 

Naturally,  the  occurrence  of  the  definite  dissociation  pres- 
sure is  connected  with  the  condition,  that  two  solid  phases 
must  be  simultaneously  present.  Due  to  the  frequently  observed 
miscibility  between  metals  and  sulfides,  the  mixtures  consist 
in  many  cases  of  a  single  phase,  a  solid  or  liquid  solution  and 

205 


206  THE  PHYSICAL   CHEMISTRY  OF  THE  METALS 

it  depends  entirely  on  the  concentration  of  these  solutions 
whether  at  the  same  temperature,  the  sulfur  tension  is  great  or 
small. 

The  experimental  material  concerning  the  sulfur  tension  of 
sulfides  is  extraordinarily  sparse.  There  is  still  difference  of 
opinion  as  to  the  series  which  gives  the  sulfides,  according  to 
the  magnitude  of  their  tension. 

The  reason  for  this  is  the  experimental  difficulty  encountered 
in  handling  the  problem,  and  the  great  influence  of  mutual 
solubility  which  the  various  authors  have  not  taken  sufficiently 
into  consideration. 

That  the  difference  of  the  dissociation  magnitude  for  dif- 
ferent sulfides  in  some  cases  must  be  very  large,  may  be  con- 
cluded from  the  possibility  of  the  so-called  precipitation  process. 
This  metallurgical  process  consists  in  heating  the  sulfide  of  a 
valuable  metal  with  iron,  whereby  the  metal  is  set  free  and 
the  sulfur  combined  with  the  iron.  Many  uses  are  made  of 
this  reaction;  for  example,  the  mercury  out  of  cinnabar  and 
the  antimony  out  of  stibnite  are  precipitated.  In  some 
cases  lead  has  even  been  obtained  from  galena  by  this 
method.  Iron  sulfide  may  also  give  up  sulfur  even  in  the 
liquid  state  by  the  addition  of  manganese  or  ferromanganese, 
to  the  melt.  This  desulfurization  process  which  is  used  in 
practice  would  not  be  possible  if  the  affinity  of  manganese  for 
sulfur  were  not  considerably  greater  than  that  of  iron. 

This  "  precipitation  process  "  has  its  exact  analogy  in  the 
reduction  of  oxides  of  noble  metals  by  less  noble,  which  has 
been  treated  in  the  fourth  chapter.  Also  in  this  case,  reaction 
takes  place  if  the  dissociation  tension  of  the  desulfurized  sulfide, 
is  greater  than  that  corresponding  to  the  sulfide  of  the  metal 
used. 

However,  the  relations  are  not  so  simple  as  with  the  oxides. 
A  series  which  gives  the  ease  of  desulfurization  of  the  single 
sulfides  is  extraordinarily  difficult  to  arrange.  That  this  dif- 
ficulty is  principally  due  to  the  solubility  of  the  metals  in  the 
sulfides  we  have  already  mentioned.  It  is  further  increased 
since  the  mutual  solubility  of  the  'sulfides  themselves  is  very 


THE  REACTIONS  OF  SULFIDES  207 

considerable  even  in  the  solid  state.  I  need  recall  only  that 
in  the  complex  diagram  of  the  nickel  matte,  the  solid  solutions 
between  nickel  sulfide  and  iron  sulfide  play  an  important  role. 
Further  it  is  no  rarity  for  the  sulfides  to  go  into  chemical  com- 
pounds with  each  other  as  we  also  recall  from  the  nickel  matte. 
The  degree  of  the  desulfurization  reached  will  under  conditions 
be  dependent  on  the  relative  amounts  of  the  sulfide  and  the 
desulfurizing  metals  that  enter  into  the  reaction. 

If  we  would  become  clear  concerning  the  yield  of  the  metal 
to  be  expected,  we  must  investigate  first  the  equilibrium  diagram 
of  the  concerned  ternary  systems.  Only  in  rare  cases  does 
the  reaction  take  place  without  matte  formation,  the  desul- 
furization being  generally  only  partial. 

We  have  now,  in  desulfurization  by  iron,  at  least  one  method 
by  which  the  dissociation  tension  of  the  original  sulfide  can  be 
considerably  depressed  and  thereby  the  energy  of  desulfuriza- 
tion essentially  raised.  This  means  is  an  addition  of  Na2S 
or  CaS  to  the  liquid  melt.  These  substances  form  with  iron 
sulfide  complex  sulfides,  so-called  sulfurets,  whose  sulfur  tension 
is  considerably  smaller  than  even  that  of  the  iron  sulfide.  This 
has  the  added  advantage  that  the  sulfide  dissolved  in  the  metal 
bath  is  increased,  not  to  mention  the  change  of  physical  prop- 
erties which  is  very  favorable.  The  sulfuret  melt  is  considerably 
lighter  and  more  liquid  .than  that  of  the  matte,  it  separates 
much  better  from  the  regulus  and  can  be  separated  very  readily 
from  it  after  cooling.  The  limitation  of  the  usefulness  of  this 
modification  of  the  precipitation  process  is  naturally  that  the 
sulfide  to  be  desulfurized  may  form  stable  complex  compounds 
with  alkali  sulfides.  The  latter  appears  to  be  the  case  with 
lead  sulfide,  where  the  experience  of  practice  shows  that  ad- 
dition of  alkali  sulfide  influences  the  precipitation  process 
adversely. 

We  recognize  from  this  that  it  is  difficult  and  not  suf- 
ficiently reliable  to  arrive  at  an  orientation  of  the  relative 
affinities  of  the  metals  for  sulfur,  by  means  of  decomposition 
of  the  sulfides  with  metals.  We  will  therefore  experiment  in 
another  way  to  get  consistency,  at  least  in  some  cases. 


208 


THE  PHYSICAL  CHEMISTRY  OF  THE   METALS 


We  saw  with  the  oxides  that  a  conclusion  concerning  the 
affinity  of  metals  for  oxygen  could  be  drawn,  if  the  equilibrium 
between  metal  oxide,  water  vapor  and  hydrogen  is  known. 
In  an  entirely  similar  way  a  conclusion  concerning  the  affinity 
of  the  metals  for  sulfur  may  be  drawn  from  measurements  of 
the  equilibrium. 

MeS+H2<=±Me+H2S. 


Ag2S+H8 
^±2  Ag-l-H2  S 


800 
700 
600 
506 
400 
300 


10      20       30 


40       50       60 
FIG.  107. 


70       80       90    H2S 


Such  measurements  have  been  made  for  several  sulfides  by 
Pelabon.*  His  results  are  given  in  the  following  table  and  the 
graphic  representation  in  Fig.  107: 


Ag2S  +H2  <=»  Ag2  +H2S. 

HgS+H24=±Hg+H2S. 

Sb2S3  +3H2  ^±  Sb2  +3H2S. 

Temp,  in 
Degrees. 

H2S. 
Per  Cent. 

Temp,  in 
Degrees. 

H2S, 
Per  Cent. 

Temp,  in 
Degrees. 

H2S,  Per  Cent. 

360 
440 
520 

6lS 
710 

21.  02 
19.85 
18.60 
17.00 
16.03 

360 
440 
520 

78.67 
85-26 
Q2.IO 

440 
510 

555 
610 
625 

44.3    !  two  solid 
48  .  6    /     phases 

*  "         two  liquod 

56-01  >        .    • 
phases 
56.9    J 

We  have  also  here  to  deal  with  the  measurements  of  in- 
complete equilibria  and  reversible  reactions.     We  find  a  definite 

*  Ann.  Chim.  Phys.  (7),  25,  365  (1902). 


THE  REACTIONS  OF   SULFIDES  209 

equilibrium  relation  of  the  two  gases  depending  only  on  the 
temperature,  so  long  as  sulfide  and  metal  are  present  as  two  solid 
or  liquid  phases.  The  latter  possibility  is  met,  as  we  have  seen 
in  chapter  3,  rather  frequently  in  systems  of  sulfides  and  metals. 
Such  a  case  of  limited  miscibility  in  the  liquid  state  is  the 
antimony-antimony  sulfide  equilibrium  in  the  above  table. 

The  percentage  of  hydrogen  sulfide  in  the  equilibrium  mix- 
ture of  gases,  as  we  see  from  our  examples,  decreases  with  tem- 
perature (e.g.,  with  silver)  or  as  with  mercury  and  antimony 
increases.  The  direction  of  the  temperature  change  of  the 
equilibrium  will  according  to  the  rules  of  chemical  thermo- 
dynamics, be  conditioned  on  the  thermo-chemistry  of  the  de- 
composition. That  side  of  the  reaction  will  be  favored  by  rising 
temperature  which  is  formed  with  heat  absorption. 

The  stability  of  silver  sulfide  accordingly  increases  with  rising 
temperature,  the  affinity  for  sulfur  increases.  With  mercury 
and  antimony  the  reverse  is  true,  an  increase  in  temperature 
favoring  a  precipitation  of  the  elementary  metals. 

To  be  sure  the  equilibrium  constants  may  only  be  used  as 
a  correct  measure  of  the  affinity  of  sulfur  for  metal,  if  the 
metal  and  sulfide  exist  as  phases  in  the  pure  state.  If  one  or 
the  other,  or  both  of  the  "  Bodenkorper  "  consist  of  solutions, 
the  hydrogen  sulfide  content  of  the  gas  phase  shows  us  only 
the  tendency  of  the  sulfide-richer  phase  to  go  over  to  a  sulfide- 
poorer. 

If  the  sulfide  and  metal  dissolve  completely  in  one  another 
the  hydrogen  sulfide  content  of  the  gas  atmosphere  depends 
entirely  on  the  mixture  relations  of  the  two  substances  in  the 
solution.  If  we  investigate  solutions  of  increasing  sulfide 
content  at  constant  temperature,  the  hydrogen  sulfide  con- 
centration in  the  gas  increases  in  the  same  direction;  it  is  first 
constant  when  the  solution  becomes  saturated  with  sulfide. 
These  relations  may  be  readily  followed  with  the  system  bis- 
muth— bismuth  sulfide,  as  has  been  shown  by  Pelabon.  It 
lends  itself  particularly  to  graphic  representation  (see  Fig.  108). 

At  600°  we  have  complete  miscibility,  at  440°  the  curve 
goes  upward  with  increasing  sulfide  content  and  at  a  determined 


210 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


place,  namely,  the  limit  of  solubility,  it  becomes  a  straight  line. 
From  this  point  on  the  hydrogen  sulfide  content  is  constant. 

From  all  these  facts  we  may  conclude,  that  the  possibility 
of  determining  the  affinity  of  metals  for  sulfur  from  the  equilib- 
rium atio  of  a  hydrogen-hydrogen  sulfide  atmosphere,  which 
is  in  contact  with  a  metal  and  its  sulfide,  is  very  limited. 

The  precipitation  process  for  obtaining  metals  from  their 
sulfides  is  only  used  in  practice  in  special  cases.  The  process 
of  reduction  after  previous  roasting  is  much  more  general. 


440° 


610° 


90 

80 

70 

GO 

50 

40 

30 

-20 

-10 


10      20      30      40      50      60       70      80      90   H2  S 
FlG.  108. 

By  the  roasting  of  sulfides  is  understood,  their  oxidation 
by  atmospheric  oxygen,  a  reaction  which  can  be  generally 
represented  by  the  equation 


(Me  indicating  any  bivalent  metal)  . 

The  reverse  of  this  reaction  has  never  been  observed.  There 
occurs,  however,  sometimes  in  place  of  the  oxide,  other  oxida- 
tion products,  under  some  conditions  sulfate  is  formed.  The 
sulfatizing  roast  plays  a  role,  for  example,  in  the  Ziervogel 


THE  REACTIONS  OF  SULFIDES  211 

process  which  consists  in  roasting  the  argentiferous  copper 
matte  so  that  the  silver,  but  not  the  accompanying  iron  and 
copper,  is  changed  to  sulfate,  so  that  by  leaching  the  silver 
goes  into  solution,  which  is  then  precipitated  by  copper,  washed, 
pressed  and  melted  in  a  crucible.  The  copper  and  iron  remain 
as  oxides  in  the  residue.  There  is  also  no  difficulty  in  changing 
copper  sulfide  to  sulfate  and  one  observes  in  the  Ziervogel 
process  always  the  formation  of  copper  sulfate  as  well  as  iron 
and  silver  sulfate.  By  heating  to  higher  temperatures  it  de- 
composes as  does  the  iron  sulfate,  while  the  silver  sulfate  remains 
unchanged. 

We  now  come  to  the  question :  "  Under  what  conditions 
does  the  sulfide  go  on  roasting  to  oxide,  and  under  what  con- 
ditions to  sulfate?"  Frequently  the  result  of  the  roasting 
depends  on  the  temperature  and  the  composition  of  the  gas 
phase  which  is  over  the  material  to  be  roasted. 

From  analytical  practice  we  know  that  by  strong  heating 
the  sulfates  of  the  heavy  metals  are  decomposed  and  that 
thereby  sulfur  trioxide  as  well  as  sulfur  dioxide  and  oxygen  are 
formed.  This  reaction  has  also  technical  importance,  or  rather 
it  has  had,  since  on  it  depends  the  old  method  for  the  prepara- 
tion of  fuming  sulfuric  acid  and  sulfur  trioxide,  in  which  iron 
sulfate  was  heated  in  retorts  and  the  evolved  vapor  condensed 
in  prolongs  which  contained  a  small  amount  of  water. 

The  decomposition  of  the  sulfate  is  a  dissociation  process 
which  can  be  repiesented  by  the  equation 

MeS04<=±MeO+S03. 

We  may  conclude  on  the  basis  of  the  phase  rule,  that  a  definite 
sulfur  trioxide  pressure  exists  for  every  temperature.  If  a 
sulfate  is  heated  in  a  previously  evacuated  vessel,  connected 
with  a  manometer,  at  a  constant  temperature,  a  definite  pressure 
results.  Since  a  number  of  metal  oxides  favor  the  decom- 
position of  sulfur  trioxide  into  sulfur  dioxide  and  oxygen,  these 
gaseous  substances  are  found  in  the  dissociation  products  of 
the  sulfate  and  raise  the  dissociation  pressure.  This  does  not 
interfere,  however,  if  we  heat  them  with  platinum  gauze  so  that 


212 


THE   PHYSICAL  CHEMISTRY  OF  THE  METALS 


the  equilibrium  between  the  two  oxides  of  sulfur  and  oxygen 
can  take  place.  The  resulting  gas  pressure  allows  us  to  cal- 
culate the  partial  pressure  of  SOs  and  from  that  the  SOa  tension 
of  the  sulfate.  The  observed  pressure  is  equal  to  the  sum  of 
the  partial  pressures  of  the  separate  gases  present. 


and  since  the  number  of  molecules  of  oxygen  that  result  from 
the  decomposition  of  the  trioxide  is  half  as  great  as  that  of  the 
trioxide,  so  it  follows 


Recently  Keppeler,*  and  L.  Wohler,  Plliddemann  and  P. 
Wohler  f  have  made  observations  concerning  the  tension  of 
sulfates  and  measured  the  values  of  P  for  a  series  of  metallic 
sulfates. 

The  equilibrium  between  SOs,  862,  and  0,  has  been  rather 
completely  investigated  by  Knietsch,J  Bodlander,§  and  further 
by  Bodenstein  and  Pohl.||  The  equilibrium  conditions  for  the 
reversible  reaction  are 

2S03  «=*  2S02+02, 


K 


C2 


S03 


The  magnitude  for  the  constant  K  as  measured  for  different 
temperatures  by  Bodenstein  and  Pohl  are  given  in  the  following 
table: 


Temperature  in 
Degrees. 

K. 

Temperature  in 
Degrees. 

K. 

528 

1.55.10-5 

727 

3.45.10-3 

579 

7-55-10-5 

789 

1.26.10-2 

627 

3.16.10-4 

832 

2.8o.  IO-2 

680 

1.12.10-3 

897 

8.  16.  10-2 

*  Z.  angew.  Chem.,  21,  532  (1908).  t  Ber.,  41,  703  (1908). 

t  Ber.,  34  4059  (1901).  §  Z.  Elektrochem.,  9,  787  (1903). 

||  Z.  Elektrochem.,  11,  373  (1905). 


THE  REACTIONS  OF  SULFIDES 


213 


TENSION  P  ACCORDING  TO  L.  WOHLER,  PLUDDEMANN  AND  P. 

WOHLER 


Temp,  in  Degrees. 

Pressure  in  Mm. 

Temp,  in  Degrees. 

Pressure  in  Mm. 

Fe2(S04)  «=>  FejOa+3S03(S02,02). 

Al2(S04)s.  «=±  A1203  +3803(802,02). 

553 

23 

572 

28 

570 

33 

621 

51 

592 

35 

681 

120 

614 

70 

702 

180 

634 

H3 

720 

261 

650 

149 

731 

356 

660 

182 

742 

480 

680 

286 

748 

692 

690 

401 

699 

560 

, 

707 

715 

2CuSO4.  <=±  2CuO-SO3+SO3(SO2O2). 


2CuO.SOs.  <=±  2CuO+SO3(SO2,Oj). 


546 

43 

600 

62 

588 

55 

653 

98 

615 

700 

686 

123 

642 

98 

705 

139 

665  , 

130 

728 

173 

700 

233 

745 

209 

714 

324 

775 

298 

725 

460 

805 

542 

Temp,  in  Degrees. 

Pressure  in  Mm. 

ZnSOi.  <=±  ZnO  -r-SCWSCfc.Oz). 

675 
690 

5 

6 

720 
750 

24 
61 

775 
800 

112 
l89 

214 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


From  the  plotting  of  log  K  the  values  for  the  intermediate 
temperatures  can  be  obtained.  (Fig.  109.) 

If  we  rearrange  the  equilibrium  conditions  somewhat  we 
obtain 


C2 


S03 


K 


i.e., 


Cso        —T' 


If  we  designate  the  S03  concentration  corresponding  to  the 
dissociation  tension  by  Csoa,  the  equilibrium  between  the  solid 


-l 


-2 


-4 


500C 


60.0 


700°  800 c 

FIG.  109. 


900° 


products,  metal  sulfate,  and  oxide  and   the  gases  SOs,  862, 
and  O2  requires, 


Cso,= 


I  c0, 
VT- 


If  the  expression  on  the  right  is  greater  the  oxide  changes  to 
sulfate,  if  it  is  smaller  the  sulfate  decomposes. 

The  same  conditions  also  hold  for  the  roasting  process. 
By  roasting  an  atmosphere  is  obtained,  which  contains  SC>2 
and  the  unused  oxygen.  •  If  we  designate  the  concentrations 
with  Cs02  and  Co2,  the  conditions  for  the  formation  of  oxide  are 


SO,- 


C/  ^02 
so2"V/^T- 


THE  REACTIONS  OF  SULFIDES 
The  conditions  for  the  sulfatizing  roast  are: 

C 


215 


It  is  frequently  more  convenient  to  use  the  partial  pressure 
of  the  gases,  in  place  of  the  concentrations,  these  are  obtained, 
expressed  in  atmospheres  (which,  however,  are  easily  converted 
into  millimeters,  by  multiplying  by  760)  by  the  general  gas 
equation: 

=  c.R.T. 


The  roast  reactions  are  of  the  form 


(oxidizing) 


<pso3  (sulfatizing). 

where  R  =  0.0821,  the  general  gas  constant. 

The  partial  pressures  of  SOs  for  single  important  sulfates 
can  be  calculated  from  the  above  given  observations  of  L. 
Wohler,  and  his  co-workers,  and  are  given  in  the  following  table. 

SOa-TENSION  OF  SULFATES 


p  SOs  IN  MILLIMETERS  FOR  THE  SYSTEM. 


i  emperature  in 
Degrees. 

Fe2(SO4)s;    Fe2Os. 

CuSOij 

(CuO)2SOs. 

(CuO)2SOa: 
CuO. 

ZnSO4ZnO. 

550 

12 

27 

600 

22-5 

29 

29 

650 

6l-5 

40 

33 

700 
7<?O 

244 

99 

35 

"?2 

I 
8  7 

800 

I  O4 

28  <; 

In  special  cases  the  roasting  process  can  be  carried  on  so 
that  the  metal  results  directly.  This  very  remarkable  reaction 
plays  an  important  role  in  the  smelting  of  copper  and  lead. 
The  metal  is  due  to  a  reaction  of  the  roasting  products  with 


216  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

the  still  unattacked  sulfide.  We  designate  this  reaction  com- 
monly as  the  "  Roasting  Reaction."  The  chemical  process  for 
the  formation  of  blister  copper  out  of  copper  sulfide  of  the  copper 
matte  can  be  represented  by  the  following  reactions: 


Cu2S+  2Cu2O  =  6Cu+S02, 


And  for  the  lead  "  roast  reactions  "*  the  process  is  com 
monly  formulated 


These  reactions  appear  very  simple.  It  has,  however,  been 
found  that  the  reactions  between  the  factors  can  under  some 
conditions  go  in  other  directions.  One  of  these  deviations  is 
of  practical  importance  in  the  Huntington  and  Heberlein  lead 
smelting  process.  From  lead  sulfate  and  lead  sulfide,  lead  oxide 
is  formed  according  to  the  equation: 

PbS+3PbS04  =  4PbO+4S02. 

The  conditions  are  still  further  complicated  by  the  condition 
that  metallic  lead  absorbs  S02  at  high  temperatures  as  Jenkins 
and  Smith  have  found,  and  that  these  reactions  give  different 
kinds  of  products,  oxide,  sulfate  and  sulfide. 

All  the  conditions  indicate  that  the  named  reactions  are 
reversible,  and  that  measurable  chemical  equilibria  occur. 
This  assumption  has  proved  entirely  correct  and  we  are  now 
able  to  give  the  conditions  under  which  the  various  reactions 
go  on.  We  will  now  show  by  an  example  how  the  laws  of 
chemical  equilibria  are  used  for  practical  questions. 

The  phase  rule  is  first  used  to  determine  the  kind  of  the 
equilibrium.  We  have  systems  in  which  there  are  three  com- 

*  For  a  consideration  of  the  lead  "roast  reactions,"  see  Ber.,  40,  2185  (1907) 


THE  REACTIONS  OF  SULFIDES  217 

ponents,  lead,  sulfur,  and  oxygen.  Beside  the  gas,  sulfur 
dioxide,  we  may  expect  as  separate  solid  or  liquid  phases,  metallic 
lead,  lead  sulfide,  lead  oxide  and  lead  sulfide. 

Whether  beside  these  still  others  as  basic  sulfates  are  possible 
needs  a  special  investigation,  whose  results  we  will  consider 
further. 

If  we  would  orient  ourselves  concerning  such  complex  reac- 
tion systems  it  is  best  to  first  investigate  the  univariant  equilibria. 
Since  a  gas  phase  is  present  in  our  system,  it  corresponds  to  a 
definite  pressure  dependent  only  on  the  temperature.  For 
the  occurrence  of  a  univariant  equilibrium  in  a  three-component 
system  the  simultaneous  presence  of  four  phases  is  required, 
that  is,  three  solid  or  liquid  phases  beside  the  gas  phase.  If 
the  three  solid  or  liquid  phases  be  heated  to  a  high  temperature, 
gas  is  evolved  and  the  reactions  are  stopped  at  a  definite  pres- 
sure if  observable  equilibria  occur. 

The  four  solid  or  liquid  phases  which  we  know  can  be  com- 
bined in  four  combinations  of  three  and  so  we  obtain  three 
possible  univariant  equilibria.  These  must  be  first  investigated 
as  to  whether  they  give  measurable  pressure  values.  The 
possible  combinations  are  the  following: 

(1)  PbS04,  PbS,  Pb,  Gas. 

(2)  PbO,  PbS,  Pb,  Gas. 

(3)  PbSO,  PbO,  PbS,  Gas. 

(4)  PbSO,  PbO,  Pb,  Gas. 

It  has  been  shown  that  measurable  equilibria  occur  in 
the  first  two.  Especially  in  the  first,  the  equilibrium  pressure 
is  arrived  at  quickly  and  sharply,  and  is  independent  of  the 
direction  from  which  it  is  approached.  The  gas  evolution  by 
heating  a  mixture  of  lead  sulfide  and  lead  sulfate  in  the  presence 
of  some  metal  comes  to  a  halt  at  the  same  S02  tension  as  the 
absorption  of  the  gas  by  metallic  lead  in  the  presence  of  sul- 
fide and  sulfate. 

The  reaction  between  sulfide  and  sulfate  begins  to  go  appre- 
ciably at  550°.  At  this  temperature,  the  evolution  of  SO2  is 
perceptible.  At  600°  measurable  pressures  are  reached,  which 


218 


THE   PHYSICAL   CHEMISTRY  OF  THE  METALS 


grow  rapidly  with  rising  temperature  so  that  by  about  725° 
the  pressure  of  the  atmosphere  is  reached. 

Sulfide  and  oxide  do  not  react  so  easily,  it  is  first  possible 
to  ascertain  traces  of  862  at  650-660°.  The  following  table 
shows  the  equilibrium  pressure: 


I.     PbS+PbSO4.  <z>  2Pb+2SOj. 

II.     PbS+sPbO.  <z±  Pb+S02. 

Temp,  in  Degrees. 

Pressure  in  Mm. 

Temp,  in  Degrees. 

Pressure  in  Mm. 

600 

39 

692 

6 

615 

61 

712 

14 

635          • 

98 

733 

23 

665 

201 

7Si 

39 

695 

402 

776 

60 

713 

590 

800 

99 

723 

735 

824 

276 

If  the  second  reaction  is  not  carried  above  800°  it  appears 
reversible.  By  falling  temperature  and  the  absorption  of  SO2,' 
we  arrive  again  at  the  pressure  values  which  had  been  found  for 
the  reaction  between  oxide  and  sulfide. 

If  the  solid  mass  is  heated  very  hot  and  then  quickly  cooled 
to  the  given  temperature  it  does  not  return  to  the  original 
condition.  We  obtain  after  the  first  absorption  of  sulfur  dioxide, 
final  values  for  the  pressure  which  are  far  higher  than  those 
given  above.  These  phenomena  cannot  be  dependent  on  a  slow 
attainment  of  the  equilibrium  from  this  side,  since  we  obtain 
it,  also  if  we  remove  the  gas  with  a  mercury  pump  and  allow 
the  solid  substances  to  evolve  new  sulfur  dioxide. 

The  position  of  this  value  in  respect  to  the  above  given, 
can  be  seen  from  the  graphic  representation.  (See  Fig.  no.) 
It  gives  the  new  curve  which  lies  between  those  for  the  systems 
sulfate,  sulfide,  metal,  gas  and  oxide,  sulfide,  metal,  gas.  What 
substance  has  been  formed  by  the  high  heating  and  quenching, 
cannot  be  said  from  these  considerations,  this  question  requires 
a  special  investigation.  We  can  only  assume  that  at  high 
temperature  the  lead  oxide  goes  over  to  the  liquid  state  and 


THE  REACTIONS  OF  SULFIDES 


219 


that  by  long  heating  it  forms  sulfate,  by  action  of  the  SC>2. 
Out  of  this  sulfate  containing  melt  we  may  precipitate  by  cooling 
different  kinds  of  products,  there  may,  as  a  single  possibility, 
be  solid  solution  or  compounds  between  oxide  and  sulfate 
present  in  the  solid  phase  and  the  variation  of  the  gas  evolu- 
tion may  be  due  to  these. 

These  we  can  only  decide  by  investigation  of  the  crystal- 
lization diagram  of  the  system  lead  oxide-lead  sulfate.     The 


700 


500 
400 
300 
200 
100 


S03  Pressure 
m  m. 


600°  650°  700°  750°  800°  850°  900° 
FIG.  no. 

equilibrium  diagram  of  this  pair  of  components  is  shown  in 
Fig.  in.  From  this  the  remarkable  fact  is  apparent  that 
there  are  a  number  of  compounds  between  lead  sulfate  and 
lead  oxide.  Two  of  them  are  recognized  by  maxima  in  the 
crystallization  curve.  They  are  the  two  basic  sulfates 
PbO  •  PbS04  and  2PbO  •  PbS04.  Beside  these  there  is  still  a  third 
compound,  3PbO-PbSO4,  whose  maximum  is  not,  however,  to 
be  observed,  since  it  decomposes  before  it  is  reached. 

The  position  of  the  melting  point  for  the  components  and 
their  compounds  are  as  follows: 

PbSO4 1 100°  PbS04  •  PbO 966° 

PbO 882°  PbS04-2PbO 95° 


220 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


The  compound  PbSO^PbO  decomposes  above  880°  into 
the  compound  PbSO^PbO  and  melt. 


Eutectic  points  are  present  in  three  cases: 

PbO— PbS04-3PbO  at  820°  and  87  per  cent  PbO. 
PbS04-2PbO— PbSO4-PbO  at  940°  and  553  per  cent  PbO. 
PbSCVPbO— PbSO4  at  950°  and  30  per  cent  PbO. 

Besides  these  there  occur  two  horizontals  in  the  diagram 
which  correspond  to  transitions  in  the  solid  state.     The  one 


THE  REACTIONS  OF  SULFIDES  221 

at  845°  corresponds  to  a  transition  of  the  pure  lead  sulfate 
while  the  second  at  450°  corresponds  to  a  rearrangement  of  the 
compound  PbSO4  •  2PbO.  This  rearrangement  is  reversible. 
It  shows  a  halt-point  with  both  rising  and  falling  temperature. 
Whether  this  is  a  change  of  modification  or  the  chemical  reaction 


2PbS04  •  2PbO  <=>  PbS04  •  PbO  +  PbSO4 


cannot  be  decided  easily  and  for  our  special  purpose  it  makes 
no  difference. 

The  temperature  field  of  550-900°  is  the  only  one  of  impor- 
tance to  us,  inside  of  this  we  have  to  do  with  three  solid  phases 
due  to  the  fact  that  three  basic  sulfates  of  lead  exist.  We  may 
conclude  from  this  test  that  the  raised  sulfur  dioxide  tension 
which  occurs  with  mixtures  of  oxide,  sulfide  and  metal,  previ- 
ously heated  to  melting  is  due  to  the  presence  of  these  basic 
sulfates  and  indeed  it  turns  out  that  the  deviation  is  conditioned 
by  the  formation  of  PbS04-PbO  and  that  the  curve  represents 
the  equilibrium. 

2(PbSO4-PbO)+3PbS  <=»  7Pb+5SO2. 

The  formation  of  this  basic  sulfate  can  be  easily  understood. 
The  action  of  862  on  molten  lead  oxide  is  under  all  conditions 
to  form  sulfate.  If  we  allow  such  a  melt  to  crystallize  we 
obtain  a  eutectic  which  consists  of  basic  sulfates.  For  the 
greatest  sulfur  dioxide  tension,  that  substance  is  necessary 
which  with  sulfide  will  develop  the  highest  pressure  and  that 
is  naturally  the  sulfate-richest  substance. 

The  pressure  value  and  curves  for  the  other  basic  salts 
must  be  obtained  by  special  experiments.  A  glance  at  the 
diagram  shows  us  that  to  procure  the  melts  which  would  allow 
us  to  measure  the  862  tension  of  the  equilibrium  system,  basic 
salt,  sulfide,  metal,  gas,  would  require  special  .preparations. 
The  measurements  must  always  be  carried  out  at  temperatures 
not  exceeding  830°. 


222 


THE   PHYSICAL   CHEMISTRY  OF  THE   METALS 


The  Structure  Constituents  Present 
in  the  Solid  Mass. 

1.  PbS04-3PbOwithPbO 

2.  PbS04  -  2PbO  with  PbS04  •  3PbO 

3.  PbS04  •  PbO    with  PbSO4  •  2?bO 


4.  PbS04 


with  PbSO4PbO 


The  Composition  of  the 
Melts,  PbSO4,  Per  cent. 

25 

25-39 
39-60 

60 


For  every  basic  salt  we  would  expect  a  curve  which  repre- 
sents the  equilibrium  between  each  of  these  salts,  sulfide  metal 
and  gas. 

Masses,  formed  by  melting  oxide  and  sulfate,  in  the  ratio, 
which  No.  3  of  the  given  table  represents,  are  rich  in  the  basic 
sulfate  PbS04-PbO  and  quickly  reach  equilibrium  with  sul- 
fide, metal  and  862  and  a  definite  862  tension  is  obtained  which 
increases  with  rising  temperature.  With  the  oxide  rich  melts 
in  which  the  other  basic  sulphates  are  contained  equilibrium 
is  reached  only  with  difficulty  and  the  position  of  their  cor- 
responding equilibrium  curves  is  uncertain,  we  are  therefore 
limited  in  our  considerations  to  the  normal  basic  sulfate.  The 
dependence  of  the  S02  tension  on  the  temperature  is  given  in 
the  following  table: 


Temperature  in 
Degrees. 

Pressure  in  Mm. 

Temperature  in 
Degrees. 

Pressure  in  Mm. 

681 

16 

78o 

217 

7l6 

42 

795 

306 

741 

81 

810 

440 

759 

130 

821 

548 

770 

184 

830 

710 

If  we  compare  these  values  with  those  of  the  system  ILz 
Fig.  no,  we  see  that  these  fall  completely  together  with  the 
curve  for  the  basic  sulphate.  By  the  action  of  S02  on  lead 
oxide  during  the  cooling  basic  sulfate  has  formed. 

We  obtain  accordingly  three  equilibrium  curves  (if  those 
for  the  di-  and  tri-basic  sulfates  were  known  there  would  be, 
in  all,  five)  their  comparative  position  is  shown  in  Fig.  112. 


THE  REACTIONS  OF  SULFIDES 


223 


These  curves  divide  the  coordinate  plane  into  four  different 
fields,  inside  of  these,  different  possible  reactions  take  place 
and  different  end  products  remain.  The  following  table  gives 


800 
700 
600 
500 
400 
300 
200 
100 


Temp. 


600°  650°  700°  750°  800°  850°  900°  950° 
FlG.  112. 

us  a  general  view  of  the  reaction  system,  the  stable  solid  pro- 
ducts are  in  heavy  type: 


Field     I.      (a)  2Pb+2S02 
(>) 

M 


=PbSO4+PbS, 

=  2(PbS04-PbO)+3PbS, 
=  2PbO+PbS. 


Field   II.      (a)  PbS04+PbS 


(c)  3Pb+S02 

Field  III.      (a)  PbSO^PbS 

(b)  (2PbSO 

(c)  3Pb+S02 


=  2  (PbSO4  •  PbO)  +3?bS, 
=  2PbO+PbS. 

=  2PbO+PbS, 
=  7Pb+5S02, 
=  2PbO+PbS. 


224  THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 

Field  IV.      (a)  PbS04+PbS  =  2Pb+2SO2, 

(b)  2(PbSO4-PbO)+3PbS  =  7Pb+5SO2, 

(c)  2PbO+PbS  =  3Pb+S02. 

In  Field  I  sulfate  and  sulfide  do  not  react  with  each  other, 
in  Field  II  the  basic  sulfate  does  not  act  on  the  lead  sulfide 
and  in  Field  III  oxide  and  sulfide  are  compatible. 

It  can  now  be  easily  shown  that  the  action  of  sulfur  dioxide 
on  lead  oxide  always  gives  these  stable  products.  Out  of 
Eqs.  a  and  b  of  Field  I  it  follows  by  elimination  of  the  metal 

4S02  +4?bO  =  3PbSO4  +PbS. 

The  same  end  products  are  left  by  the  basic  sulfate  with  S02. 

In  Field  II  one  obtains  the  basic  sulfate  by  the  action  of 
sulfur  dioxide  on  sulfate,  as  well  as  on  oxide,  and  in  Field  III, 
the  reaction  takes  place  to  which  one  ascribes  the  principal 
role  in  the  Huntington  and  Heberlein  process: 


The  stable  products  always  result. 

It  is  easiest  to  reach  the  most  stable  state  by  the  action  of 
sulfur  dioxide  on  the  metal.  The  progress  of  the  reaction  with 
time  can  be  followed  very  conveniently  by  means  of  a  manometer 
and  further  there  is  the  possibility  by  variation  of  the  gas 
pressure  and  temperature  to  start  from  every  point  in  the 
field. 

The  experiments  on  the  reaction  of  metallic  lead  with  S02 
in  Field  I  are  easily  understood.  Their  results  are  shown  in 
Fig.  113.  Metallic  lead  absorbs  S02;  if  we  observe  a  closed 
volume  of  this  gas  in  contact  with  lead  at  constant  temperature, 
we  notice  a  decrease  of  the  gas  pressure  to  a  constant  value 
which  represents  the  equilibrium  of  sulfate  with  sulfide  and 
metal.  The  values  653°  :  101  mm.,  690°  :  240  mm.,  fall  exactly 
with  the  sulfate  curve,  observed  from  the  other  direction. 

If  the  experiments  be  continued  still  further  so  would  a 
further  decrease  in  pressure  occur  after  a  time,  especially  if 
the  sulfate  undergoes  rearrangement  to  basic  sulfate  by  tran- 


THE  REACTIONS  OF  SULFIDES 


225 


sitions  in  the  solid  phase.     When  the  next  curve  is  reached,  the 
pressure  would  remain  again  constant  for  a  time.     Also  this 


arrest  is  finally  passed  and  if  the  amount  of  lead  reacting  is 
very  large  the  pressure  sinks  to  very  small  values.  Such  a 
complete  experiment  would  require  considerable  time;  it  would, 


226 


THE  PHYSICAL  CHEMISTRY  OF  THE  METALS 


however,  be  excellent  to  determine  the  position  of  the  resulting 
equilibrium  at  the  considered  temperature.  Reactions  which 
are  connected  with  the  evolution  and  absorption  of  gases  are 
best  investigated  at  different  temperatures  by  the  surveying 
of  the  reaction  isotherms  and  from  the  position  of  the  pressure 
halt-points,  the  equilibrium  diagram  is  constructed  in  an  en- 
tirely similar  way  as  we  put  one  together  for  solidificatio*-  and 
transition  processes  from  the  temperature  halt-points. 

Since  we  have  not  a  complete  absorption  diagram  at  our 
disposal  we  will  have  to  be  content  with  single  sections  of  such. 
A  closed  volume  of  862  shows  in  the  presence  of  lead,  which 
has  previously  been  treated  with  lead  sulfide  at  800°,  the  follow- 
ing change  of  gas  pressure  with  time: 


Time  in 
Minutes. 

Pressure  in 
Mm. 

Time  in 
Minutes. 

Pressure  in 

Mm. 

Time  in 
Minutes. 

Pressure  in 
Mm. 

O 

745 

96 

135 

132 

125 

22 

559 

IO2 

134 

192 

no 

24 

403 

108 

132 

228 

IOO 

42 

251 

114 

131 

72 

156 

1  20 

I2Q 

84 

142 

126 

130 

465 

40 

The  halt-point  lies  inside  of  the  Field  III,  somewhat  above  the 
value  which  the  pure  oxide  shows  at  800°,  it  belongs  accordingly 
to  the  oxide  rich  basic  salt,  probably,  however,  the  lead  is  still 
not  sufficiently  saturated  with  sulfide. 

In  all  the  above  treated  equilibria  the  metal  phase  consists 
not  of  pure  but  of  sulfide  saturated  lead.  If  the  sulfide  amount 
present  does  not  reach  saturation  equilibrium  may  be  obtained; 
this  is,  however,  not  a  univariant  but  a  divariant  system,  and 
the  SO2  pressure  depends  on  the  sulfide  concentration  of  the 
lead  bath. 

In  Field  IV  equilibrium  curves  do  not  exist  and  it  is  more 
remarkable  that  we  find  a  halt-point  there,  as  the  experiment 
represented  in  Fig.  114,  shows.  At  880°  is  shown  a  retardation 
in  the  decrease  at  a  pressure  of  90  mm.,  at  940°  in  the  interval 
470  mm.-430  mm.  It  is  certainly  not  to  be  treated  as  a  univari- 


THE  REACTIONS  OF  SULFIDES 


227 


ant  equilibrium.  There  is  formed  on  the  upper  surface  of  the 
metal,  first  a  relatively  concentrated  solution  of  sulfide  in  metal 
which,  due  to  its  lower  specific  gravity,  floats  on  the  surface. 
This,  in  contact  with  lead  oxide,  gives  a  sulfur  dioxide  tension 
which  remains  till  the  concentration  of  the  solution  is  decreased 
by  diffusion  into  the  lower  layers  and  since  the  diffusion  equaliza- 
tion requires  some  time,  the  retardation  is  easily  understood. 
Also  in  Field  4,  the  reaction 


=  2PbO+PbS 


60       120      180      240      300  Minutes 
FIG.  114. 

can  take  place,  this  is  true,  however,  only  so  long  as  the  solution 
is  not  saturated.  At  the  high  temperature,  it  is  further  to  be 
considered  that  one  no  longer  has  a  solid  oxide  or  sulfate  phase, 
but  a  liquid  solution  of  sulfate  in  oxide  whose  concentration 
depends  on  the  external  conditions.  In  any  case  we  see  that 
to  get  an  understanding  of  so  great  a  multiplicity  of  phenomena, 
as  the  reactions  between  the  components  of  the  ternary  system, 
lead-sulfur-oxygen  show,  the  consideration  of  the  solubility 
of  the  sulfide  in  the  metal  and  the  miscibility  of  oxide  and  sulfate 
in  the  molten  state  is  absolutely  necessary. 


NAME   INDEX 


Allen,  H.  S.,  35 
Arrhenius,  S.,  42 

Bancroft,  W.  D.,  134 

Barus,  C.,  5 

Bauer,  116,  118,  119,  120,  126 

Baur,  162 

Benedicks,  84,  85,  96 

Berzelius,  i 

Bijl,  60 

Bodenstein,  212 

Bodlander,  212 

Bornemann,  122,  125 

Bornstein,  9,  10,  41 

Boudouard,  168 

Bredig,  40 

Broniewski,  16 

Burgess,  n,  15 

Byers,  37 

Bunsen,  R.,  181 

Carpenter,  84 
Chamberlain,  9 
Charpy,  16,  65,  69 
Chretien,  116 
Cohen,  13,  17,  18 
Crow,  15 

Darrin,  37 
Davy,  i 
Day,  no,  113 
Dean,  R.  S.,  35 
Desch,  ii 
Dewar,  13 
Diesselhorst,  24,  78,  80 


Drude,  23,  24,  25,  26,  28,  30,  32,  33,  34, 
78,  80,  81 

Ewen,  9 

Fabre  du  Faure,  181 

Falcke,  162,  177   185 

Faraday,  22 

Findlay,  134 

Foerster,  84 

Franklin,  37 

Franz,  23,  26,  77 

Friedrich,  114,  115,  118,  123,  124,  128 

Getman,  17 

Gibbs,  134,  135 

Glaesner,  162 

Goerens,  87,  88,  89,  90,  91,  92,  93,  94, 

95,  96,  97,  99,  ioo,  102,  103,  104, 

105 

Goldschmidt,  149 
Goubau,  8 
Graham,  80 
Grenet,  16 
Grunchant,  116 
Griineisen,  78 

Guertler,  9,  17,  74,  75,  107,  133 
Guldberg,  152 

Hagen,  29 
Hanke,  37 
Heberlein,  216,  224 
Heller,  169,  177 
Henning,  157 
Heraeus,  57,  180 
Heusler,  62 


229 


230 


NAME  INDEX 


Heycock,  45,  53 

Heyn,  54,  84,  no,  112,  113,  116,  118, 

119,  120,  126 
van't  Hoff,  45,  156 
Holborn,  no,  113,  157 
Humphrey,  16,  84 
Huntington,  216,  224 
Houghton,  9 

Jager,  24,  78,  80 
Jenkins,  216 
Johnston,  4,  5 
v.  Juptner,  143 

Kahlbaum,  4 
Keppler,  212 
Kinder,  15,  20 
Knietsch,  212 
Koessler,  37 
Krafft,  6,  28 
Kurbatoff,  95 

Landolt,  9,  10,  41 

Langmuir,  I.,  8,  35 

Lea,  Carey,  39 

LeChatelier,    16,  54,  74,  84,  140,  145, 

147,  151,  152,  154,  158,  164 
Lehman,  18 
Leroux,  114,  115,  118 
Levin,  60 

Lewis,  141,  143,  168 
Lichtenberg,  68 
Liebenow,  77 
Lorenz,  40 
Lowenstein,  156 

Maey,  70 
Martens,  54 
Matthiesen,  74 
Meyer,  G.,  41 
Meyer,  V.,  5 
Meyerhoffer,  9,  10,  41 
Mittasch,  202 
Moissan,  4 
Murray,  9 
Mond,  202 

Nernst,  21,  143,  155,  158,  169 
Neville,  45,  53 


Newton,  68 
Nichols,  37 

Osmond,  84,  109 

Parkes,  47 

Pattison,  52 

Pelabon,  116,  141,  208,  209 

Peltier,  77 

Petersen,  106 

Pluddemann,  212 

Poggendorff,  181 

Pohl,  212 

Preuner,  159 

Puluj,  80 

Puschin,  72 

Ramsay,  43,  44 
Randall,  168 
Rathke,  84 
Rayleigh,  77 
Reinganum,  22 
Rhead,  164 
Richards,  T.  W.,  135 
Roberts- Austen,  41,  84 
Romanoff,  47 
Rontgen,  124 
Roozeboom,  51,  84 
Roschdestwensky,  31 
Rosenhain,  16,  84 
Roth,  4 
Rubens,  29 
Rudolphi,  131 

Schaum,  13 
Scheele,  140 
Schenck,  169,  170,  177 
Schule,  F.  A.,  77 
Schuster,  30 
Semiller,  177 
Siedler,  4 
Smith,  216 
Smits,  169 
Sorby,  54 
Spring,  47 

Ssaposhnikow,  68,  69 
Stahl,  143,  149 
Stansfield,  84 


NAME  INDEX 


231 


Stas,  3 

Stead,  127,  128 

Tammann,  45,  54,  57,  63,  107,  121,  131, 

i33 

Thomson,  J.  J.,  23 
Thomson,  W.,  20 
Treitschke,  121 
Turner,  9 


Waage,  152 
Warburg,  6 


v.  Wartenberg,  155,  160,  6 

Wheeler,  164 

Wiedemann,  G.,  23,  26,  77 

Winkler,  i 

Wohler,  i 

Wohler,  L.,  212,  148,  141 

Wohler,  P.,  21 

Wood,  68 

Wiist,  84,  85,  103,  106 

Zeemann,  31 
Ziervogel,  211 
Zimmermann,  170 


SUBJECT   INDEX 


Absorption  of  dyes,  28 

metals,  28 

Alloys  between  sul fides,  123 
binary,  49 

coincident  melting,  60 
density  of,  70 
electrical  resistance  of,  73 
freezing  point  of,  44 
hardness  maxima  in,  69 
heat  conductivity  of,  77 
low  melting,  68 
magnetic,  62 
.    maximum  melting,  60 

microscopic  examination  of,  55 

minimum  melting,  60 

of  metals  with  metallic  compounds,  82 

sul  fides,  114 
potential  of,  72 

relation  of  physical  properties  to  structure  of,  68 
segregation  in,  52 
structure  of,  53 

temperature  coefficient  of  electrical  conductivity  of,  75 
ternary,  64 

Aluminum  sulfate,  sulfur  trioxide  tension  of,  213 
Analysis  of  blast  furnace  gases,  182 

graphite  and  amorphous  carbon,  178 
Aniline  dyes,  absorption  of,  28 

luster  of,  27 
Antimony— antimony  sulfide,  115 

explosive,  17 
— manganese,  62 

sulfide-hydrogen,  208 
Arsenic — alloys,  125 
— copper,  129 
—iron,  129 
— lead,  129 
— nickel,  131 

233 


234  SUBJECT  INDEX 

Arsenic — silver,  129 

— zinc,  129 
Austenite,  95,  96,  97,  101 

Barium  peroxide — barium  oxide,  146 
Blast  furnace  processes,  180 

gas,  182 

Bearing  metals,  69 
Binary  alloys,  49 
Bismuth — antimony,  58 

— bismuth  sulfide,  209 

— lead-tin,  65 

—zinc,  47 
Boiling  points  of  metals,  4 

Cadmium — mercury,  61 

—zinc,  49,  55,  56 
Calcium  plumbate-plumbite,  147 
Carbon — iron,  83 

— precipitation  of,  .in  iron-carbon  alloys,  98 
Carbon  monoxide,  catalysis  of  37,  164,  166,  270 
— iron,  173 
— iron  oxide,  162 

reduction  by,  159 
— zinc  oxide,  160 
Catalysis  and  passivity,  37 

of  carbon  monoxide,  37,  164,  166,  170 
Cementite,  85,  87,  88,  89 

— iron,  85 

Charge  on  one  electron,  25 
Chromium — tungsten  steel,  108 

influence  of,  on  iron  carbon  alloys,  104 
steel,  1 08,  109 
Coefficient  of  absorption,  28 

temperature,  of  conductivity,  31 
Coincident  melting  alloys,  60 
Colloidal  solutions,  aqueous,  39 

solid  or  molten,  40 
Colored  metals,  34 
Color  of  metal  vapors,  6 
Cooling  curves,  10 
Component,  135 
Compounds,  intermetallic,  60,  61,  63 

with  metallic  properties,  82 
Conductivity,  21 

relation  between  heat  and  electrical,  23 
Constantan,  76,  77,  81 


SUBJECT  INDEX  235 


Copper — antimony,  74 

— arsenic,  129 

— copper  oxide,  no 

— copper-sulfide,  116 
matte,  123 

— silicon,  132 

— silver,  51 

Critical  solution  temperature,  47 
Crystal  growth,  20 

Crystallization  diagrams,  of  solid  solutions,  57,  58,  59 
compounds,  61 

Decomposition  of  oxides  by  heat,  148 

Degrees  of  freedom,  135 

Density — change  in  transition  of  tin  14 

change  on  melting,  9 

of  alloys,  70 

of  explosive  antimony,  17 
Desulfurization  with  iron,  207 
Diffusion  of  metals,  41 
Distillation  of  metals,  3- 

Elastic  limit,  68 

Electrical  conductivity  of  solutions,  21 
gases,  22 
metals,  22 

properties  of  metals,  21 
Electromagnetic  theory  of  light,  29 
Electrons,  concentration  of,  in  metals,  30 

normality  of,  in  metals,  31 
Electron  theory,  22 
Enantiotropy,  18 
Equivalent  weight  of  electrons,  24 

Faraday's  law,  22 
Ferrite,  91,  92,  93 
Form  changes,  32 
Frauenhoffer  lines,  7 
Freezing  point  depression,  45 

of  alloys,  44 
Fusion,  heat  of,  9 

Gases,  blast  furnace,  182 
Gold — nickel,  60 

—platinum,  57,  59 

— silver,  58 

— sodium,  54 
Goldschmidt  process,  149 


236  SUBJECT  INDEX 

Graphical  representation  of  three  component  systems,  66 
Graphite  and  amorphous  carbon,  analysis  of,  178 
— iron,  101 

Hardness  of  alloys,  69 
Heat  conductivity,  23 

conductivity  of  alloys,  77 

of  transition,  18 

specific,  ratio  of  at  constant  pressure  and  at  const,  vol.,  6 

of  fusion,  9,  10 

.  decomposition  of  oxides  by,  148 
Heraeus  process,  57 
Heusler's  alloys,  62 
van't  Hoff's  equation,  156 
Hysteresis,  108 

Index  of  refraction  of  metals,  28 
Iron — arsenic,  129 

carbide,  151 
— carbon,  151 
— carbon  monoxide,  173 
— iron  sulfide,  1 20 

magnetism  of,  16 

modifications  of,  15,  83 

oxide-carbon,  177 

oxide-hydrogen,  159 

passivity  of,  34 
— phosphorus,  127 
— silicon,  142 

sulfate,  sulfur  trioxide  tension  of,  213 

transition  points  of,  15 

volume  change  of,  transition,  15 

Kinetic  theory,  24 

Kish,  98 

Kurbatoff  reagent,  95 

Law  of  mass  action,  152 

Wiedemann  and  Franz,  28 
Faraday's,  22 

LeChatelier's  principle,  151 
Light  absorption,  27 

reflection,  27 
Lead— arsenic,  129 

— bismuth — tin,  65 
— bismuth — zinc,  47 
— lead  sulfide,  114 


SUBJECT  INDEX  237 


Lead — sulfate  basic,  222 

—sulfur— oxygen,  218 
Luster  metallic,  21,  28 

Magnetism  and  passivity,  35,  36,  37 

of  iron,  16 
Magnetic  alloys,  62 
Manganese— antimony,  62 

— effect  on  cementation  of  iron,  198 
— effect  on  steel,  106 
Manganin,  76,  77,  81 
Martensite,  90,  91,  96 
Mass  Law,  152 
Mattes,  123 
Melting  points — definition  of,  7 

of  metals,  8 
Mercury— oxygen,  141 

—cadmium,  61 

— sulfide-hydrogen,  208 
Metallic  luster,  28 

solutions,  39 

Metal— oxide — oxygen,  140 
Metals,  reduction  by,  149 
Miscibility  of  metals,  46,  57 
Molecular  weights  of  dissolved  metals,  44 
Monatomic  state  of  metal  vapors,  5,  6 
Mond  nickel  process,  203 
Monotropy,  18 

Nickel— arsenic,  130 

— gold,  6 1 

—nickel  sulfide,  122 

—silicon,  133 

—steel,  108,  109 

Normality  of  electrons  in  metals,  31 
Newton's  metal,  68 

Oxygen  tension,  141 

of  metal  oxides,  143 

Erin's  methods  for,  146 
Optical  constants  of  metals,  21,  27,  28 
Osmotic  pressure,  42 
Oxides,  alloys  of  metals  with,  no 

Palladium — oxygen,  144 
Parkes  process,  47 
Passivity,  37,  36,  35,  34 
Pattison  process,  52 


238  SUBJECT  INDEX 

Perlite,  93,  94,  95 
Permeability  and  passivity,  35 
Pest,  the  tin,  12 
Phase  Rule,  133-138,  140,  150 
Phosphorus  alloys,  125 

— copper,  126 

— iron,  126 
Potential  of  alloys,  72 
Polymorphy,  n 
Precipitation  process,  207 

Radiographs  of  gold — sodium  alloys,  54 

Ratio  of  heat  conductivity  to  electrical  conductivity,  24 

Rates  of  reaction,  201 

Reactions  of  sulfides,  205 

Reagent,  Kurbatoff's,  95 

Reduction  by  gases,  149 

CO,  159 

H,i5o 

metals,  149 
Reflection,  light,  27 

constant,  28 

Resistance,  electrical,  of  alloys,  73 
Roast,  sulfatizing,  210,  211 
Roasting  process,  210 

reactions, '216 
Rontgen  rays  in  the  investigation  of  alloy  structure,  58 

Segregation  in  alloys,  52 
Silicides,  131 
Silicon — copper,  142 

effect  on  steel,  106 
—iron,  132 
—nickel,  133 
Silver — copper,  51 

—oxygen,  113,  141 
— silver  sulfide,  118 
—lead,  51 

sulfide-hydrogen,  208 
Speiss,  129 

Solidification  curve  of  binary  alloys,  149 
Solutions,  metallic,  39 

colloidal,  39,  40 

solid,  crystallization  diagrams  of,  57,  58,  59 
electrical  conductivity  of,  21 
vapor  pressure  of  metallic,  42 
Sorbite,  95 
Steel,  chromium,  108,  109 


SUBJECT  INDEX  239 


Steel,  high  speed,  no 

tungsten,  108 

chromium  tungsten,  108 

nickel.  108,  109 
Sulfatizing  roast,  210,  211 
Sulfides,  alloys  between,  1 23 

of  metals  and,  114 
reactions  of,  205 

Sulfates,  sulfur  trioxide  tension  of,  213,  214 
Sulfurets,  207 
Sulfur  tensions,  206 

trioxide,  dissociation  of,  212 
Supercooling,  7,  8,  n 

Temperature  coefficient  of  conductivity,  31,  75 

eutectic,  51 
Ternary  alloys,  64 
Tension,  oxygen,  141 

sulfur  trioxide, 
Tin  pest,  12 

— bismuth — lead,  65 

gray,  12 
Transition  points,  20 

of  iron,  15 
of  tin,  14 
of  metals,  17 

Transition,  volume  change  of,  15 
heat  of,  18 
density  change  of,  14 
Troostite,  95,  96 
Tungsten  steel,  108 

Vapor  density  of  metals,  6 
pressure  of  metals,  5 

metallic  solutions,  42 
Vapors,  color  of  metal,  6 

monatomic  state  of  metal,  5,  6 
Viscosity  of  gas,  31 

Water  vapor,  dissociation  of,  155 
Woods  metal,  67 

X-rays  in  the  investigation  of  alloy  structure,  53 

Zinc— bismuth— lead,  47 

—cadmium,  49,  55,  56 

— arsenic,  129 

oxide — carbon  monoxide,  160 
Ziervogel  process,  210 


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